(A) What's the best power tool to cut paving slabs and rusted metal bolts?

http://en.wikipedia.org/wiki/Thermic_lance

Seriously though, do you mean both at the same time? A disc cutter or
angle grinder - though for optimum cutting, use different discs.

(B) What product should I use to fill dents in car bodywork?

Proper dents or just dings - and is it a valuable car or not? The way
these dent removal companies can smooth out dings is quite impressive.