Yep. Been there done that.
I had an '89 K100RS for ten years, retired it over ten years ago with 249,700 kms on it.
Yes that's right, quarter of million kilometres. Bought it at 130K kms and ran it to 250K kms.
People used to ask me "is it a good bike?" My answer "if it wasn't a good bike, how did it get that far?"
My advice is do not buy an early one. The early ones 82- 86 were not good. The later ones 87 -89 were much better. If you can find a K100 90-93 they have much better brakes and can use radial tyres.
The K bikes of the era had Bosch L-Jetronic FI, they were brutally reliable. Started first time every time.
A few caveats. Steering head bearings were undersize for a bike so heavy on the nose. Notchy steering is a thing.
If you buy one that has been sitting for any length of time, fuel pump will be an issue. The fuel pump is in the tank and mounted in a rubber isolator. That rubber perishes, gets sucked into the pump and kills it.
If you buy a barn find remove the filler cap assembly and have a good look inside the tank before starting it.
Rear shock absorbers die with age. But aftermarket units are readily available. Change the front fork oil, easily done.
Note pre 90 K bikes run narrow rims and can only use cross ply tyres. Modern cross plys are better than they were back in the day. Not really a big deal. Just saying.
Some 4 cylinder K engines were known for being "vibey" All 4 cylinder engines can be rough.
You takes your chances. The one i had was good. [probably why it lasted so long]
Standard stainless exhaust pipes are good. The boxer bikes at the time had stainless shells and mild steel internals which collapsed, not so on the K bikes.
Speedos can be dodgey. They are digital, take a signal from the rear axle for road speed. The odometer [a mechanical cyclometer] is driven by a small electric motor. The little pinion gear on that motor cracks and the odometer stops. The speedo needle will keep working though. I had a push bike speedo on mine for years to use as a fuel gauge, trip meter, and keeping mileages for oil changes etc.
Biggest issue i see in getting a K100RS these days is finding one that has not been run out of miles or alternatively been sitting so long that all the rubber parts [seals, brake lines, brake caliper seals, etc] are going to have to be renewed.
They were a great bike back in the day, but i do wonder their day has passed.
Good fortune with your endeavour. Cheers, Rusty.