Apparently used mattresses are a SIDS risk, so good idea to buy used. I myself bought a used mattress without knowing this fun fact, so there's that. (Kid's fine!)
If you're concerned about off-gassing, buy it now and let it air out for a while.
Unfortunately, I can't help you with the price point. For the record, I don't think the baby is going to find one mattress more comfortable than another. They're supposed to be hard. Also, my toddler has never complained about the comfort of her mattress, so I'm going to say as long as you find it to be a firm, safe mattress, with a water proof cover, you're good to go as long as your kid will fit on the mattress.