8mm or 10mm.
Forget powerflush anything .. they are the latest money-making scam.
Problem is, traditionally, they have to be connected to a manifold .. and this can create it's own problems when bleeding, etc.
Best to stay away from microbore and use 15/22mm copper wherever possible. Small is actually goog, as is low volume, as circulation/heating effect is quicker.
You should replace any other rads. If you do not have any complaints regarding heat outputs from present, use a similar size. Slimmest will be the single panel with convector. Available in different heights.
I would think average between £80-120 per radiator depending on location, fixing methods, etc. There is drain, flush with a cleaner, and sentinel inhibitor to protect afterwards. That could add up to £120-50 ish, I would have thought.