its really up to you. but of the members (rocking the d-series) on H-T, D-series.org, Turbod16.com; most have these items or "better". not saying its the formula for a "400 whp" d-series, but it is very close.

780cc injectors
8.5-9.0:1 CR
h-beam rods
t3/t04e 57 trim turbo
cast manifold
~28x10x3 intercooler
tial wg & bov
arp headstuds
z6/y8 headgasket
boost controller (able to adjust to ~25*)
many hours of tuning
stock otherwise

however the +500 whp d-series club has more requirments, like agressive porting or the head. so again its really up to you with how much power your going to be shooting for. but don't be surprised if you shoot for 500 and fall short.