what size of rad? 360mm should do it but you might get away with 240.
pump? D5
tubing? Whatever floats your boat
7990 block? go for an ek
what fittings? again, whatver floats your boat
fans? corsair sp 120s
Coolant? di water with additives or mayhems
Sell the H100i and buy an extra rad with the money, depending on your case it might be hard to fit both the h100i and other rad, either way you'll get better overall performance with the radspace being used for the GPUs too.
What case do you have? Will let us give some better recommendations, and by us I mean just me so far lol
Not sure it will be worth it tbh, setup will cost you quite a bit, I would only go for the water setup if you're interested in the aesthetic too, otherwise maybe drop to a high end single gpu setup, might save you money that way.