I have owned three K75's (87 K75T, 91 K75RT & 90 K75S) and two K100's (85 K100 & 87 K100), they are definitely bikes that last, and cheap to purchase.
Bosch FI on those are just simply rock solid. Performance is somewhat meh, can be a bit top heavy, but handling is still much better than most.
Consider their age, even the mid 90's models would have brake lines that are nearly 25 y-o (regardless of mileage).
When that ABS pump goes kaput, changing out the brake line, rebuilding the brake calipers will be PITA.
Early model years tend to have electronic problems with circuits in the headlight unit.
Carrying a passenger on them tend to get excessive fork dive, with 25 y-o fork seals, that can easily cause fork oil to seep out, onto brake calipers, ruin the brake caliper seals, brake pads, etc.. I know because I've done them.
Integrated luggage are nice, but add overall width to the bike.
With their age, parts are increasingly becoming difficult to get, genuine BMW parts are never cheap.
I still have a few spare side panels, wheels & brake pads in my basement. Likely sell them to Beemer Boneyard for cheap next time I visit them.
BTW, I sold my 87 K100 to a co-worker of mine years ago, he used to ride that thing everywhere along the East coast, until one day, 20+ y-o pressurized fuel line bursted while riding, caught fire while riding. :
Since that incident, I haven't purchased another 80's or 90's BMW K-bike.
As much as I like to buy cheap, reliable bikes, I hate to experience that, especially carrying a passenger.