Filter socks, skimmer, and a return pump for your sump once you get it, plus some kind of clip-on light for the refugium section.
Gonna need an extra heater and powerhead for your saltwater mixing/storage container.
If you're going to run your RODI
line straight to your freshwater storage container, you'll probably want a float cut-off valve.
I would get some extra DI
resin right away.... Depending on the size, filling both storage containers is likely going to be quite a bit of RODI
water right off the bat.
I don't know which RODI
system you plan on picking up, but if you get it from BRS on a group buy, I highly recommend the dual DI
canister upgrade.... your DI
resin will last longer. And the inline dual TDS
meter is invaluable for monitoring the health of your RO
membrane and condition of your resin.