An RO/DI would have at least 4 stages.
A prefilter, a carbon block, a RO membrane and a DI filter.
Along with that you need an inline pressure gauge and a TDS meter to monitor filter condition and water quality.
The lower the micron rating of the prefilter and carbon block, the better job they do of protecting the expensive RO membrane.
I did a quick build on BRS and it came out to $166 for the above with a 1 micron prefilter, 1 micron carbon block, 75 GPD
membrane, single DI, dual inline TDS meter, liquid filled pressure gauge and a hose bib or garden hose type connector. The smallest filter micron range they offer is 1 micron and I much prefer 0.5 microns for a prefilter and 0.5 or 0.6 microns for a carbon block.
For a quick comparison, look at the CSPDI in this sale flyer: Untitled Document
Its $199 but you get a .5 micron absolute rated not nominal rated prefilter, a 0.5 micron 20,000 gallon carbon block rather than a 9,000 gallon 1 micron version, a specially treated and batch tested high rejection rate 90 GPD
RO membrane, 20 oz of fresh specially blended SilicaBuster reef specific DI resin, a capillary tube type flow restrictor you trim to fit your exact conditions instead of a fixed non adjustable type and the same dual inline TDS meter and pressure gauge.
The difference in initial cost is soon returned in cost savings on membrane and DI replacements and you save from then on. the SilicaBuster should last 2030% longer than other resins and the membrane could last up to 10 years or more due to the better prefilter and carbon block if you stick with the recommended maintenance all vendors suggest. I would rather spend a little more up front and have lower operational costs from day one myself.
Either way, you are on the right track with getting your own RO/DI system so you alone control your water quality.