Forged pistons/Rods (Nick the site trader does a good deal on some)
Turbo/wastegate combo (to give you an idea of turbo sizing with the Garrett GT2871r .64 internally gated I made 370rwhp @1.2 bar, with GT3076 .64 and external wastegate made 476rwhp @1.6 bar, also with turbo choice you could go for a GT2871r with a .84 housing and will probably be able to crack the 400hp and have a good responsive set up)
Bigger fuel pump (Bosch 044 or similar)
Return fuel lines and fuel pressure reg
Aftermarket ecu (Depends on what your tuner is happy with, would try get one that has the ability to go MAFless, Link/Motec etc, if you get something that needs a MAF like the powerfc you may spend some time trying different MAFs to get one that will tune nice)
Intercooler and piping
Intake and filter
3" dump pipe/exhaust
While it's all apart may aswell go for ACL race bearings etc too.
If you're going for high hp you want a relatively long good flowing designed tubular exhaust manifold
A good twin plate clutch/flywheel combo (OS Giken etc)
Big injecters (I maxed out 550cc, 650cc and 750ccs before I commited and got Injecter dynamics IDA1000s (1000ccs))
Oil cooler is a good thing to have but not needed.
As much cooling/heatproofing as you can do in the engine bay.
A BOV (up to personal choice on that one but still get a well known brand as always)
As with anything there will be alot of trial and error with your build as you're trying to push it quite abit further than most people.
Moving the battery to the boot is not a silly idea either, as the turbo sits right by it which can have some not so fun consequences (My battery caught fire in Feb destroying the battery and making a good mess of the loom/relay box)
If you are going for a full wire in ECU try moving it into the cabin, two benefits 1. Gets the ECU out of a hot enviroment 2. gives you a nice place to put your intake filter if you are going MAP.