C
definately underseal. Its not expensive, and only time consuming really, perhaps a full day or two if you are a stickler like me, just get them nooks and crannies!
cambelt price sounds good, but of course you need to add on the cost of thebelt, tensioner etc, and you may as well get all the other belts changed at the same time. Its a bit of an arse of a job to do, but any good mechanic will do it just by jacking the car up, removing inner arch liner and accessing the tensioer etc from there, easy! Think its about 4 hours labour, ish, just ensure a GOOD mechanic does it, because if they f*ck it up, them bad things can ensue!
glad you got a new black one, mines black, and almost mint! where is the dent, actually on the boot lid? Might be worth considering a new boot lid, will cost you less than a repair, probably..