This is a hard question to answer as there are many cars available if you are going to consider used cars. Do you just want an appliance, like a Camry or Accord? Do you want something with some sport like maybe an A4, 3-series, or even 2006+ VW GLI (jetta version of the GTI)? Do you prefer stick or automatic? Asian, American, or European?

I will say that the G35 (and maybe the G37) sedans are pretty affordable on the used market. What about a used Mazdaspeed 3 or WRX? Of course with those cars, you probably will still get people who want to race, but they are also really fun cars.

FOr under 20k you can get into some interesting vehicles.