R1100RT things to look for.
If it is going & riding ok, the only thing to look for is the clutch. If slipping due to abuse, it takes 24 man hours to fit one. If it is slipping due to oil ingress from the gearbox pushrod seal, allow another 6 man hours to fit seals in the gearbox.
At BMW dealer rates, the above repairs are uneconomical, though easy enough to do if you are competent to do them yourself.
Most annoying feature: As sold, the engine will not run with sidestand down, even in neutral. Many are modified so that they will do so, though, like later bikes, the engine should stop if a gear is engaged.
A brilliant bike. I crossed Germany 3 years ago on one, covering 500 miles in 6 hours, stopping 3 times for food and fuel. Cruised at an indicated 120.
As anyone doing this discovers, the limiting factor is not how fast the bike will go, it is fuel consumption. Go any quicker, and journey takes longer due to additional fuel stops.
Myke