For an RO setup you can use a bucket or large storage container. I use 2 55-gallon plastic drums and a float switch to turn off and on the water. You will burn through about a gallon for every 1/2 to 3/4 a gallon you make. Mine does 200 gallons a day and I don't run it full time. I only turn the water when I need to make more water. I do keep water in the system at all times.

I use flourex about 6" above for seedlings. You don't want the internal temperature of the leaves to get to hot. Don't worry about covering the rockwool until they pop out. You give them light when you plant the seeds.