Installation is free with Sky+ but you will need to pay £99 pounds for each box so its actually 198 quid.
You can buy second hand sky boxes, but then you would be looking at an installation charge which is 60 quid as your not getting the box's from Sky, i would say Ebay is probably your best bet there.
Have a look
here..
You also would be able to get interactive sports etc, in fact all interactive stuff with an NTL line because Sky+ does not need a phone line to work (i have sky+, i have interactive services and its not even connected to a phone), it is however a requirement that the Sky+ box be conected to a phoneline in the contract however, like i said, i dont and have never had any problem.
However, i do belive that if you are using Multiroom like you intend to that Sky will check quite frequently that you are connected via a phoneline.
I dont believe that has to be BT - but im not 100%.
Hope that helps.
BTW, if i does - could i recommend you as i get another box for recommending a friend....