I watched the video on the site and read through some of your documentation. I will use the USB interface. If I read that right, I will only need the Arduino MEGA (for circuitry) and screwshield.
What determines if how many 5V DINs (if I referenced that correctly) I will need? Assuming I only need 1 12V for the MEGA. I plan to use the valves you have on the website...1x 3-way, 2x 2-way, and 3x proportional valves. I will have 3 (brewing) or more (fermentation also) temp controllers. I have two pumps already that I want figure out how to control the power to (they are 110V AC).
Not sure what you mean by DINs... but I think you mean power in. You could power the MEGA with one 12V power supply, and also use that 12V supply for other accessories, etc.