Regarding the specification to a suitable computer, would be available from many sources, but I would suggest that the help and support side could be better, and may possibly come from a local computer builder, who would be 'on hand' to solve any problems. Configuration to your computer could result in software problems, and this in turn could result in expensive software help-line support. As to actual recommendation of a computer, I will leave this to a more knowledgable person, who perhaps as a computer model type doing exactly what you require.
Looking into the ISP side, then this can be the usual hard subjective question. What one person decides another may frown. I personally have used Tiscali/Lineone for many years with satisfaction. Friends have recommended Wanadoo/Freeserve whilst others have used their cable TV provider networks.All have their problems, but on the whole, all the big boys seem to follow each other, as they are all reliant on BT services and line supply. Looking at some of the offers to hand, it would perhaps pay to select one of this trial package offers and see how that goe's, before you make a final commitment.
Always pay by credit card or interest free computer finance loan. This will give you much added consumer protection.
Sorry that I have not been more helpful on your exact requirement, but some suggestions are there for consideration.