This site has a lot of information on looking for and buying bikes.
Sometimes a bit overwhelming because he writes a lot, but I've got some
useful tips off there, especially the stuff about frame quality:
Try local free ads papers, friends and relatives and maybe asking at a local
cycling club. All of these might be cheaper than the internet.
Don't know what the market is like where you are, but it is possible to pick
up a rideable vintage bike for €100-150 where I am, especially if you're not
too fussy about the brand.