My current setup is running 179bhp but you'd be damn lucky to get all of it fitted for ~£600. Best bet if the car is totally stock would be to get some decent suspension, wheels and brakes before you even think about fettling under the bonnet. Coilovers are always a worthy investment, but they're £600 on their own for D2's. Once you've sorted that out, you will need...
FMIC
Exhaust Manifold
HKS Actuator @ 1 Bar
RRFPR
Walboro 255 fuel pump
2.25"-2.5" exhaust system + de-cat
NGK Iridium Plugs (Heat range 7)
Magnecor leads
The best part of 180bhp in a car weighing 860kg's stock is plenty fast enough to be getting on with. The mods above are also an excellent base for going down the larger turbo route. But remember, brakes and handling first!
Ah i hear that a lot on here but tbh handling and brakes are last on my list, but saying that i don't often drive hard
Yeah i honestly think that with great power you need handling and brakes. Its one thing having 180 bhp for example but he car will hanle like a dog with standard suspension and no handling mods. I think to compliment the two handling and brakes should come first.
Then again each to their own if you prefer to do mods and no handling or brakes so be it.
Last on the list?! So, you're going to increase the engine output by ~40% and not touch the stock fade-for-fun brakes? The previous owner of my car did just this. Do some heavy braking from national speed and by the second time, you've got next to no brakes. When I say "upgrade the brakes" you don't have to go out and piss £1000 on a monsterous 3G brake kit. Decent pads, lines and Dot5.1 fluid will suffice and will cost ~£200. Money well spent in my opinion, but if you've got a death wish, by all means, crack on.