Decent bikes from the as far back as the 60's mostly use standard threading and can be easily repaired with new parts. I ride a bike from 1971, it's quite light and fast, and like any bike that's properly cared for doesn't have problems with rust. $250 on craigslist.

One of the main advantages of buying used bikes is that people actually used to build bikes properly so they last 43 years with all stock parts.