I'm not 100 % certain, but i'm sure if you buy a SKY + box from Ebay/Sky or whoever. You would still need the SKY subscription (Minimum) £16.50 a month to get the record functioning (Pause - Rewind - Record etc).
That would mean you would have to buy the £16.50 1 mix deal, or I think there is a £10 subscription for Sky + full functioning alone. You might aswell pay £16.50 and get the variety pack

.
SKY + BOX - £50 - free installation
The third choice could be get a £10 Sky Multiroom room subscription. So you have the Sky + Box downstairs, with the Standard box in your bedroom.
If you subscribe to anything on Sky, you will get the Sky+ functionality free. Murdoch loves cash!
I think i'm right by the way, I guess I could be mistaken but I don't think I am.