2 things are really standing out to me here:
1) why is everyone talking about dry hopping adding more bitterness? it doesnt. to extract bitterness, you need to boil the hops to isomerise the alpha acids in the hops. dry hopping adds mainly aroma, and some taste.
2) what you are looking at making will give you an alright beer, but dont expect anything close to grolsch. for a start grolsch is a lager, not an ale. it doesnt make a scrap of difference whats written on the tin, if you want to make a lager, you need a lager yeast (pretty much all kits come with ale yeast, exception being euro lager and pilsener, which have real lager yeasts, and mexican cervesa which has a blend of lager and ale).
not sure what hops used in grolsch, but the last time i remember tasting it was a few years back, and it was pretty bland, watery and boring.
a standard coopers kit, say lager, draught, pale ale, canadian blond... whichever one, with 300-400g ldme and 600-700g of dextrose, a short boil (or dry hop if you really cant be bothered) of some nice euro hops (saaz, hallertau, saphire, tettnang etc) and mainly a decent ale yeast (s-05 is best for this) fermented nice and cool (16-18) will give you something fairly decent (and imo, far better than grolsch)