Do you already have Sky+? We didn't when we went from Sky to Sky HD, and the engineer had to add a second widget to the satellite dish and run a second cable into the house, so it couldn't have been done by just buying the box and activating the card.
yeah, we already have sky+ so its just a case of bringing the box and switching it on. Tbh i think they have a cheek charging for it, but its half price at the moment, so the same as getting one on ebay.
I'd probably go for getting it direct from Sky, as then you'll be able to get them to sort it if there are problems. I'd bet if you've just bought it from eBay and there are any problems then they'd not be very helpful! Maybe phone Sky and ask how they suggest doing it as a surprise present? They must get plenty of people wanting to do that at this time of year.