The price will all depend on what package you are currently on. Generally speaking it will be around £49.95 install / activation fee plus £11.50 a month extra for a TiVo. However if you'd be happy with a standard V HD box check out something like the Premiere Collection (
http://collections.virginmedia.com/).
It will have been activated previously but will be synched to another customers account. Therefore you would have to ring VM to synch it to your account and they will probably refuse as you've not obtained it directly from them.