what else would we have to pay for so we can watch the satelite from our apartment?
If it is a "proper" community dish system, then you should have access to all the UK TV channels. If you want more then you can pay a monthyl subscription to sky and use a sky card and get their pay tv channels.
in some cases some community systems have restricted channels, you must use the "comminuties" boxes, and you have not chance of using a sky box or sky card.
so hopefully someone here will be able to confirm which system you have
we have a spare sky box in the uk would we be able to use this in spain
See above
will we need a viewing card and if so how much does this cost/ where can we get it from?
See above.
And many UK TV channels are available for free and require no sky card to view.
however, there are some stronger ITV, C4 and Five channels, and Fiver Five USA Sky 4 that do require a freesatfromskyc ard (£20 in the uk).
what channels are included without paying a monthly subscription charge?
Again, depends on what "type" of system your community has.
Many of the main UK TV channels, and about 200 others are available for free and with no sky card - free to air channels
The stronger ITV, C4 and Five channels, and Fiver Five USA Sky 4 that require a freesatfromskyc ard (£20 in the uk) - free to view channels
http://www.satandpcguy.com/Site/freesatfromsky_tv_channel_guide_list.php
is there any other useful info we should know before saying yes?
if you want to use a PVR like Sky+ or freesat+, this requires two cables from the system to the receiver.