1: IMO, defo get a few spare diff pulley wheels, wishbones, the odd hub and maybe a rear shock tower, I'm only surmising as since I've broken nowt 'cept pulleys and shredded a couple of belts, though I am using 5.5
2: Stick with stock spur for now, pinion will depend where you're racing, cells etc, but at a rough guess, start with an 18 or something, trial and error.
3: This again depends on what surface, you are running on, does it have jumps? is it high grip? etc.
Finally: M3 capheads would be best for you me thinks, for these you'll need a 2.5 hex driver, if you go for normal button heads though, these use a 2mm hex driver. happy screwing.