It will cost you more to build your own than buying a nice reef ready system already assembled.
Take a look at the ProPlus here:
http://www.spectrapure.com/email/customer-...eciation.html#1It is almost the perfect system, the $249 MaxCap is its big brother and the perfect system.
$149 complete. Thats with a dual inline TDS meter, inline pressure gauge, hose bib adapter, clear housings, batch tested and treated RO membrane, capillary tube flow restrictor, 1 micron prefilter and carbon block, full size vertical DI filled with custom blended SilicaBuster DI resin and more. You can't buy the parts for that price much less have it assembled.
With an RO/DI you don't size them by the reef system size so much as how long you want to stand around waiting for water. Yes you could get a 15 or 25 GPD RO/DI system but the cost would be the same, the physical size is the same, it would make the same quality RO/DI water but it would take 3-5 times longer to make water so would have your faucet tied up longer if you were using it portable.
The Spectrapure 90 GPD systems make water at the rate of 3-4 gallons per hour so not as much waiting for water to fill the tank or bucket.