For a 20 gallon, a zoo-med or Exoterra UTH will be fine get one that covers about 1/3 of the bottom of the tank.

As for thermostats stay away from the ones sold in pet stores they aren't worth your money.

Herpstats are great very accurate and has built in safety features

http://spyderrobotics.com/

If money is tight these will work but should be replaced when possible:

http://www.amazon.com/Hydrofarm-MTPR..._bxgy_ol_img_b

Either way make sure you have a thermostat. Otherwise you will have a BBQ's BP