i got an 86 model k100rt with 70,000km but odo wasnt working with receipts from bmw shop for about 15 years
guessing from the tyre repacements its done 90,000
no receipts mentioned spline lube, clutch splines were almost perfect, final drive splines almost perfect
forward drive shaft splines almost perfect but rear splines about 10%wear
i got another for spares with 175,000 again odo wasnt working very neglected
drive shaft rear spline very worn but final drive good
the free play was a little more when changing direction on the rear tyre
if yours is worn im guessing u will have time to wait for a cheap repair
it seems to me that the cheaper part will wear first
i would take the risk, not waste his time, and consider 2000
maybe another test ride with basic checks like a car, head gasket, smoke,oil leaks, overheating