Don't mess around with used tires. It's not worth the risk unless you're destitute. Tires start "aging" (aka rotting) the second they come off the assembly line and even a tire that has deep tread can still be about ready to disintegrate or are so old that even if they are in decent shape you won't get much useful life out of them. You shouldn't run tires on _any_ vehicle, regardless of mileage, for more than 6-7 years.
Your best course of action is to go get a low end house brand tire (Costo, Big-o, Discount Tire, etc) which you can get for $3-400 (maybe even less if you put some effort into it). The tires are cheap because they have poor tread life, are noisy, and have relatively poor handling. Things you don't care about in an old beater that you barely drive.