you should be asking yourself how often you plan on doing water changes and how many gallons each time.... RO/DI units are rated based on GPD... gallons per day... essentially, how much clean water they will produce in a given day.
And remember when ordering from Ebay you get what you paid for. I wouldnt recommend ebay at all but thats my opinion. Try drfostersmith or petsolutions on some Rodi's