I followed you here from the "New to Chickens" thread.
Some answers to your questions:
Q. "How hard was it to install the thermostat?"
A. It was very easy. I am by no means an electrician. I just went on-line and looked up basic electrical wiring and took it from there.
Q. "Do you think I could just buy a digital house thermostat and install that inline with the power cord to the lightbulb?"
A. Again I'm no electrician. I do believe the typical house thermostat is low voltage and can't handle 110v. (If someone here knows for sure please chime in. If I find out it can be used and if I do it again I will switch to the digital.)
Here's some pics. It's been a couple of years since I used it so the styrofoam has fallen off. The large front door is for acessing and turning the eggs. The little hinged door on the bottom rear is for sliding the water pan in and out. I had to change the water every now and again or it got yucky! I only hatched 2 dozen but it is large enough to do 4.