, nitrate, phosphate, and possibly silica is what i'd test for. Also, can you use one of the powerheads that you have already as a return pump? My LFS
. Ebay has a very wide selection of RO
units as well.
, I used 1/4" thick 5/8" inside diameter clear rubber tubing for my return line with a tee connection at the end for extra directional current. It doesn't really matter what you use, but the connector is good ole hard plastic that drills easily and cleanly, to put your syphon-break hole.
And, don't forget to do some emergency situation tests such as unplugging the powerhead to see how high your sump fills up when there is no power. Good luck.