I just bought an ro/di unit last week (kent) and here are some learnings:
1) The water output rating is based on 80 lbs pressure and 80 degree water (or something like that!) So if your pressure is 40 (mine is 35) you get half, and if your temp is 60 (mine is 50 coming out of the well) you get half again. So keep in mind you could potentially get far less than the rated. So get a big one. And don't locate it under your kitchen sink, or you'll have a big water bucket sitting in your kitchen for days at a time 'filling up!'
2) The water wasted is incredible. I'm dumping at least 10 times as much water down the drain as I make. So keep that in mind as well. If this is an issue for you, consider other systems that don't create waste water (like just DI).
In my ro/di unit I actually re-plumbed it to BYPASS the ro membrane altogether! So I use the 2 pre filters and the DI filter for fish water. According to the manufacturer, this creates the SAME QUALITY water as with the RO. The downside is that the DI filter gets used up quicker, the advantage is much much faster water flow, and no waste at all.
Good Luck!