This post was my start, I got the part numbers for the Fet's I used, resistors, and diodes, and basically copied all the physical control from this post. I started with basically what he has, and then expanded it with a screen because I wanted feedback.
Fallen Tree Branch 60cm (jungle mode)
Now,... I wanted temp control, I fiddled around a bit with analog thermistors but they were inaccurate with everything else going on. Finally I found this guy, waterproof and accurate. This website is great! Not only do they sell you the part, but they provide sample code and libraries. I used their sample, and put it into my code with a minor tweak to average 20 readings before displaying, and convert to degrees F.
And since I liked that site so much I picked up this screen. I later changed that to a I2C screen but I would not recommend it, I would stick with this style if you can.
Once again, I looked at their example and copied it. There documentation is really good.
As far as controlling the fan/heater goes, I flip the fan on the same way AnotherHobby flips on his lights, and I do the same thing for the heater except that the FET turns on a relay which switches a plug on and off. I then plug the heater in there.
Hope this helps!!! If you have any questions fire away.