I dont know. IMO its a waist of time as your not going to be able to compete with
http://www.rightmove.co.uk/ for users as EVERYONE in UK knows of that URL. I am sure you can get clients with free posting but not users so where are you going to make your money? Bandwith for images are also going to be a major cost if your doing it on the cheap?
If you must do it then do narrow your niche to your local area and even so its not going to be easy.