You won't need a map sensor or a fuel pressure regulator for those power levels. 400bhp is much less than 400whp. So probably a low boost kit I would imagine.

Ecu, injectors! Fuel pump and a tune and you should be good. Stock clutch will probably hold for a while but if you have the funds I would do it now as well.
Gauges are always a good idea so you can monitor things and as mentioned above 1 step colder plugs are a good idea.

I definitely say to go for it if you have the money. It will make the car that much more fun to drive!

Can definitely keep the diff stock unless you are planning on doing a lot of launching. Mine has had zero issues at 12 psi. I have never launched my car however.
No need to change up the brakes or anything. Stock are great!

 

Guys thanks so much for the posts! They were extreamly helpful!! Going to save the thread and use every bit of advice and information you have given me!

Many Thanks

Sebby Out