I also bought an RO/DI unit from ebay for about $100 a couple months ago. it's an off-brand from a manufacturer called "abundant flow" but it seems to work fine.
it claims to produce 100 gallons in a day, but i'd estimate more like 60 or 80 if i ran it continuously for 24 hours. Which is still fine for stockpiling pure water for topoff and weekly changes. i store the pure water in a 20 gal. rubbermaid container with a powerhead and an air pump running in it. a day or two before i do a water change i mix a batch of saltwater.
when the unit came from ebay, i was definitely confused and overwhelmed. it's a really high-tech looking device and it came with sparse instructions. in practice, though, it's really simple to use. a red tube screws onto your sink faucet (input), a yellow tube goes down the sink drain (waste water) and a blue tube goes in your collection bucket (pure water). it takes about 5-6 hours to fill the 20G container, and i produce about twice as much "waste water" as "pure water." I just set it up and let it run, and try not to monopolize the sink from my 4 roommates for longer than i have to!
just make sure you order it with the right input adapter. mine came with an adapter for a garden hose, rathern than a sink faucet and i had to order the right piece separately. Oh and the valve system was very confusing also. there are two ballvalves on my unit and i just flipped them on and off in different combinations until i was getting 2x waste water and 1x pure water like i knew i was supposed to.
the filtration tubes are all replaceable on my unit and you can order backups from the company's website. (http://www.abundantflowwater.com/
). when i emailed them about my adapter problem they actually responded right away, so i guess i would recommend this manufacturer to others.
generally, look for a 3-stage unit (you really don't need more) that advertises around 100 GPD. that should be sufficient.