Carp, I think this will be fun and a great learning experience for you and "the team". I've often debated with myself about getting an automation controller for the FW, but several times concluded that I'm getting it because it would be "cool" rather than really having the need for it - for me personally anyway. IMO, there are many relatively inexpensive (compared to Apex) that exists that would more than meet my needs for FW. Temp controller, ATO, nitrate monitor, pH, etc...Again, this will be a good experience.
In my humble opinion whether for profit or for fun/learn, as with any software/hardware product you should scope out the functions and the features you want (or think people want) to have. Start with something simple like temp controller, then iterate on it. You've mentioned "design" a few times, but I'd take a step back and look at functions and features you want first before designing anything.