if you have sky dish, then you have freesat already, you can indeed pay sky for the viewing card to see all the freesat channels (think its 20quid), but not sure if your SKY box will allow recording, you have to then pay them a monthly fee for the EPG/record, think thats 11quid – but dont quote me, they change all the time, search for
‘freesat from sky’
also remember that for freesat twin recording, you usually need 2 inputs from the LNB / dish, freeview i dont think you do, just means you may have to pay out something extra if you go donw the freesat recording box.
imho, freesat / freesat HD is much better for pic quality than freeview unless you have got a super-decent ariel installed.