Holdposition
Most of what you have asked has been answered above but if I may offer some advice.
One connection per address is correct but shop around for your ADSL provider as costs and services vary. Please bear in mind however BT provide all our infrastructure (unless you have a local cable company) so as much as it pains me to say this it is better sometimes to use them as in the past I have come across users that have had passwords changed on the connection between service provider and BT which makes for an interesting couple of days.
BLUE SKY THINKER Suggests Pipex and I would have to agree however ask around your neighbours as service varies in different locations in my experience.
Because of the relatively good take-up of ADSL I think there are about to be price cuts from what I have been reading of late, this will appear in the form of faster connections at similar prices to today but means that generally prices should fall.
I have to agree with Richard that once you use ADSL the dial up will become painful to use.