It depends mate if your happy with 400 or if one day you will want to go more? , also depends on the condition of your engine too.
The engine build for 400bhp would be around £4.5k for an engine tuner build , that's drive in drive out . if you want the engine to be capable of 500+ , it's around 5k , but then you will need injectors , mapping , turbo exhaust , intake and maybe intercooler and clutch .
I have a build thread on hear , I'm running 414 hp on an enginetuner package , I'm only running a front pipe on the exhaust , a mishimoto top mount , rcm headers and uppipe and a engine tuner spec mamba turbo .,
Turbo and headers will be about £1700
+ 500 ish for injectors
+ couple hundred for exhaust depends what you go for
+ 500 ish for a new top mount , although this may not be necessary.
+ 500ish for the clutch
It can end up getting quite expensive dependant on what parts you pic