a) what size wheels could I get that wont look silly in the arches?
they are sportpack arches, so anything that is 6 or 7 inches wide
b) is there such a wheel as 13x6 deep dish to match what is on now (so I only change the front - the rear do not stick out past the wheel arches - the reason for needing to change..)
yes there are 13 x 6, the original sportspack wheels were 13 x 6, but those wheels shouldnt stick out at the front, i have 13x7 superlites on mine and they sit exactly flush.
c) do 13 (or 7) inch wheels cause more wear on the car?
yes, you should really have an uprated diff pin fitted or x-pin diff, they will cause more wear to the wheel bearings too, but these arent too bad to replace! with the width it will cause extra strain on ball joints and stuff as well as they tend to find more holes in the road! but again, arent that bad to fix/maintain!
d) any suggestions on wheels (inc tyres)
your personal choice mate! i personally like the wide wheel look, i think it looks mean! but if you want different wheels its your choice! as for tires, i really like yokos, they are worth the extra bit of money!
bear in mind your body will have been cut to fit the extra width of the wheels (on the front anyway) so i dont know what arches will fit to the car now that bits of the body have been cut away!