I use the Digital Aquatics Archon on my tanks to control functions, and while it can have bugs as it is still a newer system receiving updates, gotta say I trust them a whole lot more than a home made controller. With $1000s in fish I need something rock solid. I considered building my own controller for temp, pH and all that good stuff but at the end of the day I decided spending a couple hundred to protect thousands was worth it. Once I have it all in place it will be monitoring two pH probes, ORP, two (maybe more) temp probes, multiple leak sensors, pump control, temp control, ozone control, dosing pumps if I decide I want them and more with wireless access to the system from any computer/tablet/phone on my network as well as out of the house once I bother to get it set up.
It is a neat project you are looking at, but I'm just not convinced it is worth it. If you decide to persue it you might want to take a look over at Reef Central. They've had many builds like yours over the years.