Thank you for the excellent reply. Well, excellent except that I have more work to do...

I thought I would just clarify something about the computer PSU. I am using it because I need 5V for the controller as well as an Arduino, I want 12v for the fans and I figured it had more than enough juice to give me 24v through a boost converter. Just in case you thought I was actually using it in the PC which will feed the controller - I am not; I will be using a separate laptop. I do have a 24v PSU but I figured the controller only wanted 500ma and seemed overkill having another supply. All the grounds of 5v, 12v and 24v would therefore be the same. Does it still sound like a bad idea? Should I have separate 5v, 12v and 24v supplies?

Thank you for your input