with you wanting one swampy that has taken the total number of interested people to 36, I was only going to do 30 but have been thinking that if demand is still there I might take it to 40, the first 10 is waiting to go I just have to test the unit and finish writing up the instructions. When I have finished the instructions I'll be asking everyone to read them carefully and to decide if they have the skills to build this unit, remember that to have a working controller will require 240V wiring and this wiring must be checked by a qualified person, you don't want to be stuffing around with 240v if you don't know what your doing.
Swampy the size of the 12v power supply will depend on what you want it to supply, if it is just to supply the brauduino shield then it can be small like 12w or 12v at 1 amp. If you are going to use 12v pump(s) then you will need a larger wattage to drive it, my unit will have unit 1 brown pump so I went with a 24w power supply. I would think that 1 march pump would be ample or maybe 2 brown pumps, if your driving a march pump then you won't need a big 12v power supply as a march pump is 240v and you will need to wire up the relay to switch that 240v.
cheers steve