I hear you. Space determined what my final water filtration system would be.
I purchased a two filter 10" cartridge system with filters that do an incredible job of filtering the water that I will be filtering, which, for the most will be municipal water.
I fitted a brass garden hose connecter on both ends and then fill my entire water tank with filtered water. I am drinking straight from the tap and the wife notices that the water is less hard in the shower.
Your space will dictate if you go 1, 2 or three cartridges.
I used this website to buy my housing and filters.
http://www.filterwater.com/c-12-countertop-filters.aspx