GravelCruiser:
I'm a road rider first and a reluctant dirt rider. GS is a really nice paved road option. I had 4 BMW RTs before my current and first GS(A). I really like RT's but the new ones are off the rails w/dumb tech.
Since you're sitting it out for a while, I decided to go long on this post. Hope you have a little fun reading it. Have to edit to 10,000 letters.
Late, non-servo Hexhead is the very oldest I'd consider. A good wet-head 1200GS or GSA, would be the best value for money..but more money.
Hexhead uses rocker-arms with eccentrics on the ends..no shims...just adjust carefully with feeler gages and tighten with a 10mm box wrench and you're done. I like this way of valve adjustment.
Cam-head uses overhead cams with shims to set valve clearance. Lots of dirt bikes use shims.
The boxer twin engine design is sensitive to the throttle bodies and valve clearances being out of synch. Earlier 1100s and 1150's had a poorly designed intake butterfly system and cable linkage. It went out of synch easily. TB bearings start to leak air at about 20K. Extra air on one or both sides throw off the synch and make the bike run rough. My 1100, was a bear to keep in the happy place.
All but a few early 1150's had dual plugs on each cylinder. This finally solved the 1100 F.I. surging issue, Great, but BMW put a terrible servo brake system on that bike...Really irritating...and they kept the same dumb throttle body setup. I had an 1150 and sold it after 2 or so years with only about 30K on it.
I put an Ohlins suspension on my 1100 and then had it fit to the 1150. Great shocks. They wouldn't fit the Hexhead, so I included them in the 1150 sale. Recommend a good aftermarket suspension on any pre-Wethead/ESA BMW bike.
Bought an '05 Hexhead RT new and put about 90K miles on it. I did most of the maintenance on it. It was a GREAT bike. Wilburs suspension was installed right away so I'd get the benefit...Nice improvement. I recommend Beemershop in CA. They ask a million questions about your weight, if you ride double, how you ride etc. What I got from them was perfection.
Farkles are expensive for BMW bikes. Buying a bike that's already kitted out is a big plus. New aluminum side-cases cost a grand, for instance, could be included on your used purchase. Vario cases have their charms too, btw. They can be extended wider when you need to pack more stuff in them. When you don't, a narrow profile fits between things and saves a few mpg at speed.
I know you're gadget-adverse but sport touring is not dirt riding. There are gadgets worth having.
I'd buy a pre-TFT bike if looking for value. Not significant extra function to TFT...just prettier.
GPS is great for touring. Garmin Nav V and VIs fit into a BMW cradle on liquid cooled GS and RT. Nav V tries to do too much with a slow chip. I'd avoid it. The Nav VI is better, but the screens fail after a season of use. Call Garmin service. They're nice as pie, send you a new one when they receive your old one. You pay freight 1 way. I'm on Nav VI #3. This last time, they told me that they'd changed the screen hardware to fix the problem. It has worked for several months now and 10,000+ miles with no hiccups. I'm hoping this is my last replacement. If your newly purchased bike doesn't have the BMW mount. Get a Garmin Navigator XT or the new XT2 with larger screen. These XT units are quite good, have fast new chips and don't break...or find an old Zumo 550...not as good but cheap and tough. Make sure any used Garmin is registerable with Garmin. Owner should transfer it to you on the Garmin site.
Cruise Control: I was against it and didn't use it for 6 months on my '05, then tried it on a long trip. I won't buy another bike without it now. Throttle-locks are just not as good...at all. This is a very good thing to have on a street bike. Not only is it great for saving gas on long trips, it is great for holding your speed down in speed zones etc. Wonderful.
ESA: It just works. On LC bikes it keeps the rear spring pre-load at optimum regardless of the load. You get some modes etc. I just set mine the way I like it and use that for everything.
ShiftAssist is a really good tool once you train yourself on its use. The newer ShiftAssist is less fussy to use, but even old, fussy version is terrific. Just normal riding in traffic, it is of great utility. Bombing corners in NC? It's priceless. 70mph approaching a 20mph corner? Just shut the throttle and press the shift lever 3 or 4 times and you're in the perfect gear with little use of the brakes. It's quite fun and quick once you get the knack. No more awkward fiddling with the clutch while you're pressed into the bars braking hard.
Get GPS, Cruise, ESA and ShiftAssist. I spelled out why but had to edit. Trust me, you want this stuff.
I would equip any road bike with a touring screen, a good aftermarket seat (I've used Sargent because they're decently comfortable, not a hassle to buy, wear well, look OK and they allow you to move around on the bike rather than sit in one static position all day (ala Russell Day-Long); they re-sell pretty well. Get the grippy seat covering. It's way better for textile riding pants.
A touring windscreen should be short enough to see over, tall enough with enough laminar flow to create a bubble of still air over your helmet, narrow enough so your arms get some air. I'm using a V-stream Deluxe on my GSA with Aeroflo Aerowings. I install my wind protection and new seat then go on a trip before deciding on other things. Usually, not always, I'll want bar-baks. These move the bars higher up and back towards me about 1.5". I can control the bike while standing on the pegs quite easily. This is good to do every 30 minutes or so of riding to keep the blood moving around below the belt. I can sit on the rear part of the seat where it's wider and more comfortable and still sit straight up on the bike. With a touring screen, I don't lean into the wind. (It's also not pelting my face shield and jacket with bugs.) Sitting up straight is kind of the right way when there's no wind pressure.
Brakes: My '05 RT had Gen2 servo brakes. They're best brakes I've seen on a BMW bike. It could panic-stop with 2 fingers on the lever, could be brought to a full stop smoothly. -1150 Gen1 servo brake bikes could not smooth stop. Gen 2 servo brakes still work with the engine off. 1150 Gen1 servo brakes barely work at all without engine power. Many near drops and one actual drop because of these dumb brakes.
As good as Gen-2 servo brakes are, many owners neglected brake fluid changes. When you buy used, you don't know if fluid was changed or how often. When servo brakes fail, they're expensive to fix. There are ways to de-servo the brakes and make them standard, but it's a lot of work. I'd shoot for a bike with non-servo to be safe. That would be late-hexhead at the oldest.
Maintenance on German Stuff is different. The internal parts and even the bolts on these machines are beautifully made. Things work in a precise way and maintenance is not for gorillas. Use torque wrenches. There are bolts that screw into very fine aluminum castings and you do not want to over-tighten them. Again, ask me how I know. Take the time to learn to do things right. It's a joy.
BeemerBonyard.com is a great source for the right oil, the right filters, the right plugs etc. They even kit the parts for a service for your particular model and year. Their stuff is considerably cheaper than the dealer charges. Save your receipts and proof of each service. Ask to see that if you buy from a private seller.
I have a friend who recently traded his white '09 GS in to Cycle Werks of Barrington, IL for a new GS Rallye. It was a sweet machine with aluminum wheels, not spoked; street tires; Sargent seat, touring windscreen and a few other nice trinkets. Someone bought a very nice one-owner, completely dealer serviced and asphalt ridden GS that, to my knowledge had never even been dropped on a driveway. I'm sure it's gone by now. That dealer gets nice used bikes. They clean them all to look like new. They fix obvious problems and test ride every one. They refuse to take a lot of bikes in trade because they won't sell bikes with obvious problems. I think they offer a short guarantee on their BMW used bikes, but I'm not sure.
I change all the fluids on used bikes right away myself. Unless it's from a BMW dealer and it's a BMW bike, I wouldn't trust that they put the correct anything in the bike.
1100's, 1150's, hexhead and camhead 1200's had dry clutches, separate oil for the engine and a different oil for the gearbox. The newer liquid-cooled 1200 and 1250 bikes have wet clutches (spinning in oil like a dirt or sport bike clutch). The gearbox is lubed by engine oil...so only one oil for clutch, gearbox and engine. There were problems with 1100 and 1150 engine output seals leaking onto the clutches and ruining them. BMW finally got smart and followed standard motorcycle design on that. I think the new way is better. It's certainly easier to maintain the bike.
A BMW bike is full of oil when, when a room temperature bike on its center-stand is about ¾ to the top of the red circle on the site glass. No more than that at room temp. When oil gets hot, it expands and this level will go up. You don't want it over the top of that red ring. Half way is OK. ¾ is OK. ¼ and the engine starts to make more noise. I add oil if I hear that...but just a little. Using oil between changes hasn't happened either of my BMW liquid cooled bikes.
Shop well, find one, make it your own and learn to take care of it and make it work for you. It's all a great and fun education.