anything chromoly butted steel with horizontal dropouts is good, but for a nice one you will have to hunt for a deal and even then, you're just a hundred or so dollars less than buying a nicer, new frame.

honestly the best 70 dollars i ever spent was on a complete although krylon'd univega conversion. that was a long time ago and i doubt you'll find a deal like that again that isn't stolen or extremely dangerous/broken.

600 dollars later i had a MUCH nicer bike built up myself and i could have just spent 300 and had something 80% of the way there.

conversions are iffy. you want a complete.