Mike brings up a good point - knowing what you need to filter out is important. For example, here in Saskatoon we deal with chloramines which takes some consideration.
My go to for RO/DI stuff is Bulk Reef Supply, but with the Canadian dollar in the toilet it's harder to buy in USD. I am still using my Kent Barracuda 4-stage RO/DI that I bought in 2004 from J&L Aquatics. All I've ever replaced on it are filters and the pressure gauge. I've modified the heck out of it, but it still functions the same as a stock unit.
For filters, you can't beat Bulk Reef Supply because they have better quality sediment and carbon blocks (smaller micron) than I've found from Canadian suppliers. I'm on a Filmtec 150 GPD RO membrane that I bought from BRS which I'm only flushing 1:1.75 (using a 75 gpd restrictor) that's been running since Apr 2010 and I'm still getting only 2 ppm out of the RO membrane (source water is 150-210 ppm). I flush it for at least a few minutes after every use, and I've been known to forget it "flushing" for an hour or two.
Interestingly, the original 75 gpd membrane that came with the Barracuda was used very lightly (had a nano tank) and only lasted me 3 years before it was producing 12 ppm. I use the current 150 gpd membrane fairly heavily and it's twice the age!
I added an HM Digital DM-1 dual in-line TDS meter and I have the "IN" measuring just
after the RO membrane, and the "OUT" measuring the product water after the DI stage. It's unfortunate that you can't calibrate these probes separately, but so far mine read the same (I put them both on the same line to check). This set up tells me when the tds is creeping up from the RO membrane and it's time to change, and also tells me when the product water reaches 1 ppm and it's time to change out the DI resin.