I would be surprised if Sky would do the installation even if you offer to pay.
About 4 months ago my mum gave me her Sky + box (only 6 months old).As an existing Sky Customer I thought it would be just a matter of contacting Sky to se how much they charge for an engineer to come and set up the connection.Sky flatly refused to do this as I did not purchase the box direct from them, and they told me I would either need to buy a new box from them or pay a local company to do the installation for me, which would cost more than Sky doing the instal even f I bought a new box from them.
At the moment I am using my Sky plus box as a standard digi box, but I suppose I will have to buy a new box from Sky if I want Sky +
As already mentioned. you will need to upgrade your Sky card, whch can be done over the phone (this is about the only thing Sky would willingly do for me)