I foolishly parted with my K100RS and have subsequently been looking for a replacement.  My previous bike had less than 50 k miles on the clock when it went to the new owner, so it was running essentially as a new machine - which is to say very well!  I now have my sights on a '85 RT with high miles. With good maintenance, my expectation is that it still is capable of many more miles of reliable running.  Is that a reasonable expectation?  
Approaching 100 k miles, I expect the following components will need or have been rebuilt/repaired: fuel pump, coolant/oil pump, alternator (?), starter (?), fan, clutch, instruments.  This list assumes that the normal wear items have been attended to in the course of the machines life ( tires, cables, brake pads and calipers and possible m/c for clutch and brakes).  
I grant you that I've posed a very speculative situation.  I feel competent in my ability to evaluate the state of the machine during a hands-on inspection, what I do not know is if a k-bike valve train, rings and transmission are essentially finished at 100 k miles. 
Thanks for your thoughts.

My RT I bought at 36k miles and its now just short of 90k miles.

In that time in no particular order I have done most of the things you mention...I have them up on the forum here.

Shortly after I got it but I new they were likely candidates and price reflected it:
Water/oil pump
Brakes rebuild
Head bearings
Cables because I wanted to, clutch was actually frayed
Rear shock

Over time:
Clutch friction plate/rear main seal
Final drive big bearing
Alternator monkey nuts
Injectors cleaned, one replaced.
MAF
Temp sensor
Stand assembly
Cooling fan

Lots of the usual items of maintenance done but some things not done because not needed

Fuel pump
Starter [opened and checked]
Brake discs
Radiator
Wheel bearings
Fork seals.

Splines at 78k miles were perfect and good for many more miles. The clutch friction plate had less than 5k miles left in it.

Cosmetically its not bad but that's the best I describe it as. But it has never let me down.

If it was mine I would be replacing the chain and guides and inspecting the valves. More for peace of mind than expecting anything to need replacement.

I didn't mention parts on the shelf, Rick G correctly mentions a very important one. Full timing chain kit is on the shelf with tensioner sprockets etc as one of those tasks to go in soon.