Pick a mission. You can't have it all. You said you want a cheap plane that can use a 500' field.

But you also want:
-- 1500 pound gross
-- Corvair power
-- Usable cross-country performance

If you really want the first, forget other three. Look for a used Kitfox 3 with a 582 rotax. I see them all the time on Barnstormers under $20K. Otherwise, pick the latest Kitfox you can afford and practice your short field technique. Any of them will technically land in 500' (for that matter, so will an RV!), but nothing over 1200 pounds with a decent cruise is really going to be SAFE on that short of a field unless you're a helluva pilot. When you can routinely land in 250 feet, then you're ready for a 500 ft runway.