I try to avoid this sort of thing as much as possible by a thorough inspection prior to purchase. Bearings can be an issue, but most bearing realted problems can be revealed by removing the wheels and chain so you can spin the axles and spindle by hand. If it looks like the replacement hardware will be hard to come by, I pass on the bicycle, or use the rough bearings (or anything I find) to justify a lower price. Once you demonstrate any problems to the owner, they usually are very receptive to a lower offer.

Often, bicycle parts are standardized enough, that modern substitutes are available. But if you're a stickler for originality or if you need something oddball, there may be quite a wait to find what you need. I'm sure many of us have projects awaiting those special parts. However, I'm not averse to putting a modern front wheel on by 1961 Legnano, so that I can ride it pending my obtaining the proper, Legnano engraved, Campagnolo hub.