heres my quick input.
Are you talking 400-480rwkw or crank kw? Don't forget the more power you want the bigger turbos you'll need = more lag....
BOV's, stock ones are fine - Leewahs car runs 600rwkw on stock BOV's.
Spark, probably worry about that once you get over 350-400rwkw.... can probably leave it until it needs it, you'll know.
power that things start breaking? tough one depends how hard you drive it, but at a guess around 350-400rwkw things would start to break..
those N1 turbos are probably good for around 500-600hp at the crank at a guess.
Yep i'd get an oil cooler, keep those oil temps nice and low.
Baffle kit/sump ext probably worth it. depends how much track work you'll be doing.
bearings not sure about brands etc... but i belive it does make a difference...
Head, thats up to you, I wouldn't stuff around with the flow of it, maybe have it cleaned up, as in pulled down replace anything that looks a bit worn etc...
Most of us just run pulp (premium unleaded petrol - 98RON here, think you may have different ratings in the states), race gas you'll get a heap more timing/power out of, but since you're not into competition i would stick to pump gas.
hope theres at least one thing here that helps