you won't need a new alt, the stock one is fine. and doing the big 3 with that amount of power won't make any difference, at least anything noticable, if that's all you're running you are fine without any upgrades.
get a dash kit for a s10 blazer, latest model, should have a DIN and 1/2 like you want. that way it might look more flush, only problem is it prob won't fit. or, PIITG