With used bikes it's about 2 things:
1. Fit. If the bike doesn't actually fit you then everything else is irrelevant. You're 6' so looking at a large or 20-22" frame. You may be able to make an 18" frame work but it still needs to be comfortable. The only way to find out is to go sit on the bikes and ideally give them a quick ride. You shouldn't feel bad about declining to buy a bike because it's too small. Don't take the owners word on how it fits too because some people have horrible setups.
2. Condition. The bike should be in a useable or easily fixable condition. Check that the rust isn't too bad, that the gears all shift, it pedals smoothly and the brakes work (pro tip: test the brakes before you go on the test ride). Good condition tyres is a bonus, but you can get tyres and tubes pretty cheaply if needed.
Ask at your local bike shop if they know of or can help you get a bike in budget, they may have or know something.
And finally, it might be worth seeing if you can get credit to buy a new one, something like a Giant Escape 3 would do everything you need without the risk/maintenance of buying something that's 10+ years old.