I am going to fit a Hotuning 6 branch manifold, but you must be careful, as they are from germany so you dont want a left hand drive one.
As for decat pipe, the come with a bore size of halfway house between an 8v and a vr6 exhaust, so it needs adapters either end to fit to the vr6 manifold and exhaust, even though it says its for a VR! (annoying i know)
I would recomend getting a full custom cat back if you are doing the mani and decat, as this will allow you to go lower and also mean that you wont be restricting the power you have unlocked.
As for back box..... up to you! lol
I made the mistake of a straight through custom from powerflow, no silencers. Way to loud, got annoying.