If you are talking budget and water cooling lights, look elsewhere for cooling. Very expensive to set up water cooled lights.
I would listen to Garden of Dreams. He builds rooms very well.
Spend your money on the things most needed. Stay away from Electronic Ballasts unless you enjoy spending money on burnt out bulbs. Get magnetic ballasts, they cost less and last longer.
If you plan on going bigger in the near future I would spend a good chunk of $$$ on your A/C. Do not buy air cooled hoods at the moment if your A/C can handle it. Buy the air cooled hoods after a few harvest when you decide to make a bigger room.
Look on Craigslist for deals on lights, you can get 1000 watters for $100-$150 if you look hard. Might even find a deal on a complete ballast/air cooled hood and bulb.
72 plants is alot for that space. You really need to know for sure of the strains you are going to go with do not stretch very much in flower.
Sometimes having less plants in a space can get you more of a yeild, or better quality. Packing too many plants in a room cuts down on light penetration. Unless you plan on running a SOG without any veg at all. That I can see.
If you are running 2 of the same hydro systems, how would you feel about splitting up the harvest. You could have a little more working room if you put one of the hydro systems in and 30 day's later put the other one in so you can harvest every 30 day's.
Not sure if you have your strains picked out or whatnot, but I would try to go with mostly Indica dominant strains. Find one that does not stretch that much in flower. Some strains can double or triple there size in flower. That is why you gotta be careful about packing too many plants in a room. You do not want to end up with a few good size colas and a bunch of fluffy buds on the bottom or sides.
With any system you go with, I hope it work's out for you. Try to keep things simple in your grow. Get a journal and write in it when you do anything to your grow. This way you can alway's look back and see what did what. Or what not do again.