[71cadc10]

I believe a supercharger kit would be easiest to add at later point, due to most of these kits are 'complete' nowadays with no fab needed. Whether centrifugal or roots/tvs.

I'd oversize the fuel system now up front, including injectors. That way it's all there when you will need it.
if you are truly shooting for 700hp I would do the forged piston now.
LS9 head gaskets and ARP bolts should be fine...you can re-assign some of the money from head studs elsewhere.

for fuel, you could look at putting a dual pump system in the tank, and only use the one pump for now natural aspirated. Then wire up pump #2 on a hobbs switch once you have boost. look at 80# injectors which should do 700 on pump fuel.
at that power level I'd prefer a stable return type fuel system, so put a regulator w/ return in there now and you can boost reference it then when the time comes (add # fuel pressure per # of boost)