If my experience (and those of others in Edinburgh) is anything to go by, the contention ratio is simply not an issue. We've two computers here (a Mac and a PC) running on our home LAN with a single DSL line; frequently both are in use at the same time and we never notice any drop in speed. It could of course be that no-one else in my area has DSL, but I think that's very unlikely!
Also, BT didn't charge me either for installation or the DSL modem, as I was an existing BT Internet customer. Might be worth having a word in their ear on that one - they may do you a deal if you're "considering another provider". <img src="smile.gif" border="0">