1. Contention = "sharing", you basically share your connection with others-rarely seems to affect service though (in my limited experience). 2. It is easy and straightforward enough. I would stick to the well known brands. 3. USB as at least you can position modem to view connection L.E.D's, pci types much cheaper though. (Some S/p's throw modem in with package-shop around. 4& 5. Check "christmascrackers" link.
1) Like anything else, if there are a lot of people online, your speed will pyramidically decrease although this is not too noticeable. If you have cable BB and change to landline you will probably find that it is slightly better, esp. if you are on NTL ;-)).
3) I use both and there is no difference that mere mortals will notice. USB 2 is terribly over-hyped unless you are doing a Spielberg with the video films.
4) Tiscali and, IIRC, pipex have no activation fee. There are many others.
5) Cheapest is not necessarily the best. I use Tiscali and it is excellent in both service and cost. It is not worth quibbling over a couple of quid a month. From experience NTL is total pants. Service from the same provider seems to vary from area to area. It is worthwhile remembering that ALL BB, except cable, is supplied by those fun-lovin' guys at BT, the name on the box means de nada.