I love the BRS systems. They are mid-range price wise, but really decent quality.
I like the 5 stage PLUS system. You will want the added items that the plus comes with (flush valve, pressure gauge, and TDS
meter). If you buy a cheaper system, eventually you will want to add these items anyway.
5 Stage PLUS RO/DI System - 75GPD - Bulk Reef Supply
Personally, I'd avoid the cheaper systems that are out there, like some of the economy systems on ebay. Have heard a lot of bad things about the waste water ratio, seals, filter life, filter replacement, leaks, etc.