I use a 5 stage system and it works great. Here is a good deal on a brand new one from Bulk Reef Supply (ARC sponsor): 75 GPD 5 stage system It comes with everything you need including filters and parts to hook it into your house's water system.
You can also buy RO/DI systems used, but unless I know the buyer I tend to avoid it as you won't know how old the filters are a could easily spend the cost of the unit replacing the filters if need be.
Most people mount their RO/DI systems in their garage as its easy to keep a 55gallon drum and mix salt in the garage. I put a Y-fitting like you would see on an outside garden faucet with one side going to my washer, the other side going to the RO/DI unit. I ran the output line from the RO/DI out into my yard to water my lawn. All summer long the parts of my grass that the output hose could reach, were green...everything else died!
For people who live in an apartment, they tend to mount them in the washer/drier room and put the waste water from the unit either into the washing machine or down the drain line.
I can help you hook it up and as I said above, I can show you mine in person as I think you are close to me. (I was confused by RO systems until I saw one in person)