All RO/DI can be stored in a closet or somewhere and don't have to be plumbed into your house. BRS offers a faucet adapter for like $6.99 and that is compatible with all RO/DI to my knowledge. People normally plumb it because its one less hassle that you have to do before preforming a water change.
Personally I would look to Spectrapure as they really are the top of the line RO/DI manufacturers out there. They just recently released a new membrane with a guaranteed 99% rejection rate, most others are 98%. Now I know that doesn't sound like much but it will make your DI last longer thus being more cost efficient. You can grab a refurbished 90GPD RO/DI from Spectrapure for $129.99 plus taxes and shipping, I don't think your really gonna be able to find a better deal short of finding one of Kijiji or Craigslist.
People usually go with a 90GPD instead of say a 40GPD because the 90's offer a higher rejection rate (98%-99%). It won't hurt the RO/DI to only make 20 gallons of water instead of 90, just so long as your not making small amounts like a gallon or two here and there.
