Thanks for all the advice everyone!
Not counting removing some oxidation from a battery compartment I did my first repair yesterday.
A Ricoh KR5 came in the mail, described by the seller to be in "good condition".
The viewfinder was black. And the shutter wouldn't fire. Film winder stuck.
It turned out the curtain had stuck in it's down position, with the mirror in the up position. Which explained the black viewfinder.
I gently helped the curtain to complete it's cycle, which resolved the stuck shutter and film winder.
The mirror had been glued to the top of the body by the decaying foam buffer at the top and looked nasty when I freed it.
So during an hour last night I cleaned the mirror and replaced the foam buffer. Good thing I had all the tools and materials I needed at home.
The view finder is still a bit on the dark side, but the camera seems to work now. I'll run a roll of film through it and see.
And some day I might have to investigate why the shutter had gotten stuck, if it happens again. But I did a hundred cycles yesterday and it never showed any problems. The times sound reasonably right as well, but I don't have any tools to measure them yet.
Also I'm curious what a camera that is in a worse than "good condition" from that seller looks like. :-)