I'm a Thomas-Stuart/Skutt fan. I've got 11 of them. They run smooth, have the most torque for the money, and the big splash pans keep the studio much cleaner.
For used wheels, just make sure it runs smoothly, and check the belt for wear. Beyond that it's hard to tell if it will run for 20 years or break down in a week. Avoid the Brent wheels with the black control box with smooth/flush buttons. Those were not good- Brent was replacing them if they died, but I don't know if they still are.