As has been (partly) indicated, if you're using an old Sky box you either need an old viewing card (or though some models do seem to work without them) or a 'Freesat from Sky' card. (£25, direct from Sky, by calling 08448 244 400).
Any generic satellite receiver will receive all of the channels listed here:
http://en.wikipedia.org/wiki/List_of_free-to-air_channels_at_28%C2%B0E
(That includes a Sky box with an old card in it). 'Freesat from Sky', which isn't exactly the same as 'Freesat', simply adds in a few extra 'red button' channels and interactive facilities. True 'Freesat' actually only gives you a subset of the channels in the list I've linked to (plus red button channels and the Freesat programme guide).
Brand new generic receivers don't cost very much more than secondhand Sky boxes. e.g. this one is £26.10
http://cpc.farnell.com/ross/dvbs-ro/satellite-receiver-fta-sd/dp/AP02506?Ntt=AP02506