Yeah I would definitely go with a Thermostat.

I picked up a few last month on eBay, Zilla 1000 Watt ones, for $25 - $30 each. I even got one for $20 shipped. They're a cheaper on/off type, not proportional, but for just a heating pad they should do well, better than a dimmer at least.

As for measuring temperatures, I'm not sure I would trust anything less then a Temp Gun. Regular thermometers are made for air temps, not ground temps. You can get a nice Pro Exotics surface temperature reader for $24 shipped. I think they're a good investment in the long run.