Here's a pretty handy link for water heater size:
http://freshaquarium.about.com/cs/tipsa ... ersize.htm
Most people recommend something called a Stealth heater, they are unbreakable and keep good, consistent temps. You'll find after pricing water heaters and brands that pretty much all of them cost less than $30, so don't feel bad splurging on the high end ones!
Thermometer, get whatever works. A lot of us have those stick on thermometers that go on the outside of the tank for quick reference (I have mine positioned furthest from the actual heater) and have some type of IR thermometer or digital probe thermometer for addition measurements. I have a small IR thermometer, I like it because I can measure the surface water temps, the water temps at the glass, and the basking temps.
You didn't mention filter or lighting. If you previously had lighting, it should work well on your tank. You need a UVB and a heating bulb, setup however you can fit them, over the basking area. As for a filter, you'll be wanting a canister filter for a tank that size. If you think you'll need to upgrade in the future (like if you find you have a female), you may want to go overboard at this point and buy a Fluval FX5. You can find deals on them often. A Rena XP3 or XP4 will also work with the size tank you have.