Thank you all for the suggestions, I knew you wouldn't let me down! Having the electrical components pre-built never occurred to me.
It almost makes things worse for pleading my case about building one - if the "pros" can't do it correctly, what chance would I have? Electricity runs somewhere between magnets and voodoo on my "chalk it up to dark magic" scale of ignorance.
I'll look into plug and play components, that sounds so much better than what I was originally planning.
If this idiot can make it work, I'm sure you can also. I don't know what the "pros" problem is, perhaps it's the crazy idea of building one with a quality thermostat.