Having owned a 1985 K100 and a 1992 K1100 my suggestion would be to look for a K1100. The K1100 is slightly larger but has better front forks and has the paralever final drive. These bikes are getting quite old and all the rubber parts, brake lines, fuel hoses, might be in poor condition. They have dry clutches like cars and if have been slipped a lot on starting from a stop the motor rear main could be seeping or leaking damaging the clutch. The oil filter is inside the motor and access is gained removing a small plate on the bottom of the pan. On the plus side they are both easy to work on.
I put 75K on an '85 K100, probably the most dependable bike I ever owned. If it's been stored/maintained properly, It should be fine with the low mileage.
I would take the other folk's advice though and go ahead and replace a lot of the rubber stuff, particularly brake lines. The bike will also feel a bit "dated" compared to newer bikes, but they are rock solid dependable. The '85's also had more power than some of the later models. It really just depends on what they want to do with the bike.
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