I don't know about the warranty availability - CPOs are pretty hard to come by in that price range, unless you'd be the 3rd owner. Dealers usually want an arm and a leg from what I've seen.
I can tell you one thing - don't even think about the gas mileage. I recently got 20mpg on a highway trip and that seems about normal. My everyday work commute is only 4ish miles on suburban roads (just enough to warm up the car really) and I've been getting 10ish mpg. Seriously, I haven't even seen 11. I have, however, seen 9.8. Of course, that's not why I bought the car.
EDIT: today I really tried on the way to work and I got the average mpg to read 11.0. Small victories =)