I have a 1998 R1100RT that is about to turn 42,000 miles. I've owned it for 3 years and it had 19,000 mi. when I got it. I does use some oil in between changes - maybe 16-24 oz. which is normal. There are no signs of oil residue in the exhaust and the plugs are clean with no oil signs either.
I think you'er right to be a bit concerned about the oil in the exhaust. It you are still thinking about buying this bike you might consider having it looked by a good BMW technician. It would be worth the investment upfront to have some piece of mind.
In my opinion, when I see a vehicle that is cosmetically in poor condition, I get the immediate feeling it may be the same for it's mechanical condition. It's the first impression thing I guess.