It's do-able, but technically speaking, it might not be do-able for everyone.

I'd personally make it from scratch because im good with electronics.

I'd just make a massive computer like board, with all the sensors needed, and run it off the mains (Obviously with a trasnformer)

You just need a thyristor and a humidity sensor and a 555 chip, and the rest if done with timers that you can buy from a shop. Obviously you'd need the right resistors, capacitors and transistors but it can be done.

Theres a few PCB (Printed Circuit Board) design programs out there to download.