Choosing a PID controller
It sounds like you have some things there that makes it hard to find a ready device that can do the work (but I'm not sure).

It sounds like you have to either write a PID yourself, 
or use a PID in some library (like the Arduino).

I am unsure about the stability of the Arduino solutions, will this solution still be stable after a year? Or do you need to reboot it every day?
But maybe it is good enough? (Test it for a month a see what it can do)

If you decide to write a PID yourself, the paper called PID-without-a-PhD is a good start: 


http://www.eetimes.com/design/embedded/4211211/PID-without-a-PhD or 
http://www.eetimes.com/ContentEETimes/Documents/Embedded.com/2000/f-wescot.pdf


And you also have application notes on this topic from more or less every MCU maker out there.