Scooper

You're getting good advice.

Adding to what cyclotoine said, I'd suggest keeping an eye out for frames built with double-butted 4130 chromoly or better tubing from the seventies, eighties, and early nineties. Unless you're picky about the components, look for complete bikes with mid-to-high end components. Complete bikes are almost always cheaper than buying a frameset and then buying the components separately.

Unless you're looking specifically for a short wheelbase/steep angled frame for competitive riding, or a long wheelbase/long chainstay/relaxed angled frame for touring, look for something in between (~73° HTA and STA with ~430mm chainstays) that will give you lively steering, yet be stable enough to be comfortable on longer rides.

As far as brands are concerned, I wouldn't worry too much about that as long as the construction quality is there, there aren't any dents, dings, bent forks, stuck seatposts/stems, etc., and it fits you well.

You can pick up some really nice bikes on eBay or CL for $200 - $300, which is a small fraction (ten cents on the dollar) of what you'd pay for a new bike of equivalent quality.

It's hard to go wrong with mid-to-high end American, British, Italian, or Japanese bikes from the seventies, eighties and nineties if they've been reasonably maintained.

My $.02. Good luck.