I just by if it is a racing bike, it fits and has a frame that is still in good shape( paint, chrome). Then I fixthem with 10 speed Campa which is quite affordable, and then I just ride them. As often similar bikes have been raced I try to find the corresponding shirt.
I tell people that I am a racing shirt collector, which isn't too costly, but that finding the right bike makes it an expensive hobby.
