Hiya and welcome to the forum, a place that mixes despair and realism in equal quantities!
My (limited) experience in cutting granite benchtops is that a ginder:
a) Won't cut the bench, control will be poor and straight lines (more or less) not feasible;
b) Will be unbelievably noisy, dusty, dirty;
c) The blade will sh*t itself fast cutting 50mm granite.
As I understand - and what I have done - the kitchen guys would use in this situation ... wait for it .... is a demo saw with a diamond blade. Dirty, noisy, but not dusty as it is WET cutting. So muddy instead. Heavy as well (say 10-12 kg, not sure) Great on top of a kitchen cupboard!
You can hire them from your local hire shop. Minimising the water (mud) damage will be the problem.
Others may have a different view, whattya reckon guys?