You want the best, lowest micron rated near absolute or absolute sediment filter you can afford, 1 microns would be largest I would consider with 0.5 or even 0.2 microns being better since its purpose is to protect the billions of tiny microscopic pores in the carbon block. A 1 micron or smaller single carbon block is more than sufficient if well protected by the sediment filter. A Dow Filmtec 75 GPD RO membrane is by far the most popular choice and Spectraure starts with just that then specially treats all their membranes than subjects them to testing, either in batches or individually as with their Select series. You want a full size vertical DI capable of holding at least 20 oz of fresh resin. Spectrapure does not use prepackaged or generi resins, they custom blend all their DI resins based on thousands of hours of real world and lab testing so it is only the best, freshest available.
If you are in the hobby for the long haul then Spectrapures MaxCap series RO/DI systems are the best money can buy anywhere at any price. If you are just starting and want the best bang for your buck then their Refurbished system for $125 can't be beat.