I wouldn't buy a battery powered one, but a siphon so you can just vacuum the substrate when you do water changes. It should be cheaper too.
As for the RO
brand, I would just get something that isn't terribly expensive, 5 stage ones are good. People over here buy them off ebay that are kinda no-name brands. Look around and see what your options are. The filters are measured in how many gallons of water per day they produce (gpd). Since you have a large tank, I would get one that is rated for 100gpd (meaning it would take it around 24 hours to produce/filter 100 gallons).
If you still have some questions about the RO
filter, this link
has some good info.
If you can find a test kit that uses liquid and not strips, that is best. The strips can be inaccurate.
I too would recommend sand. I know you have all those "pencils" in there that you have paid for. I might leave them in the sump, but in the main display, I would look into aragonite sand. It's much cleaner looking. I can see the "pencils" trapping waste and making your nitrates rise, even with siphoning and vacuuming.
I would look into getting more rock. If you can find live rock, I would get some of that, plus some non-live rock as well. The rock you have in the tank now is the non-live kind. Live rock basically means that is has some bacteria and probably some type of critters on it. The bacteria (which convert ammonia to nitrates, part of cycling) is the best part. You can get mostly non-live (or base) rock and eventually the base rock will become live. I would check the fish stores in the area, as the ones that sell saltwater fish and corals usually sell live rock. Live rock acts as another filtration system. Its recommended that you have 1.5 to 2 pounds of rock per gallon, but it doesn't all have to be live in the beginning and you can add it in a little at a time, if you don't have fish (sometimes it can have some die-off and release ammonia in the tank).
Your tank is really nice! Can't wait to see what you do with it.