Can�t comment on the differences between brands � I�ve only had a Pace digibox and it has been 100% reliable over about three years.
Without a card you can still get all the free to air (FTA) channels but you do need a card (20 quid from Sky, lasts 3 years, or try to get one off ebay) to get the free to view (FTV) channels as these are encrypted. They are ITV, Channel 4 and Five. But you can just as easily switch back to terrestrial to view these.
If you�re buying from ebay, check whether they are supplying a remote and a mains lead. Not all do.
I have a friend in the same position as your Dad (dish and connections already installed but no digibox) and I�m trying to persuade him to go down this route rather than buy a freeview box.