It is possible, but if you are in a hard water area with high tds then its not commercially viable, you'll be spending out every few weeks on resin. "have neither the money or space in the garage"? I live in a first floor studio flat and have found space in a cupboard for a merlin ro and 240 ltr wheelie bin. Proof that you can do it on a budget of a few hundred quid. Cant advise on water softener although I understand they are quite costly to buy, compared to a basic ro. If you really hav'nt got the space I'd consider going for a merlin in the van, you can top up through the day then if need be.
Regards
Simon