So, I decided on an arduino platform. For my requirements I needed a couple of things:
First, I needed the ability to measure at least 4 analog values from 0 to 5v.
Second, I needed at least 1 PWM output (for the future electric heater).
Third, I needed at least 3 normal inputs (ie: off, on).
Fourth, I needed one normal output for a heartbeat/status light. And four normal outputs for inside fan high/low, outside fan on, and compressor on.
Fifth, I needed a couple of data lines for future communications.
Sixth, It had to have a watchdog as well as enough memory to hold the many future upgrades planned for it.
Whew! That's a huge list!
Well, after much ado I decided on the Sparkfun ProMicro 5v 16MHz. I could have bought one of those Chinese knock-offs for a lot cheaper, but this thing needed to be rock solid. It fit into the case without too much fuss, and is well designed.
Now to put my nose to the grindstone and get to wiring and coding...