To get the pure water that is recommended for aquariums, you want an RO/DI. Units are Home Depot are usually only RO and do not contain DI canisters. You can buy a unit that is RO only and add a DI canister if you like, but you would probably save money by buying an RO/DI unit. Look online and you can find some good prices. I just got my Typhoon III and need to hook it up.
Typhoon is a good buy, i've heard several good things about them.
i've got a spectrapure, works great.
i would also recommend getting ro/di and not just ro. if it is a reef tank, i would say definitely get a ro/di unit.