helmet - obvious reasons
fire extinguisher - obvious reasons
half cage - can be lived with but carrying people in the back is a no no, stiffens chassis to an extent
full cage - expensive, pain in the ass if you're using on the road, stiffens the chassis majorly, all helps the suspension
recaros or buckets - side bolster support is a must have, needed if you want harnesses
harnesses - make a load of differnece, make you part of the car (instead of a passenger)
suspension - no sense having the power with no turn in! (include top mounts camber/caster adjustable)
brakes - decent pads and discs unless you're prepared to replace em every time they get cooked!
engine mounts - solid front one at least, you won't believe the differnece!
tyres - yoko 32r semi slicks (or 48r as they're now known) cheap items from german ebay
anti roll bars - must have suspension upgrade!
polybushes - makes everything a lot stiffer and they don't wear out all that often
perspex and weight loss - all the weight that goes is extra horses, things like sound deadening etc
decent gauges - need good feedback from the engine
close ratio box - instead of trying to put more horses through long gears, try shortening them and the horses come from nowhere! i believe you can fit a smaller final drive which does the trick. it's about £40, however the gearbox needs pulling apart
lightened flywheel - allows revs to be picked up quicker and so improves acceleration
baffled sump - to keep a good amount of oil available through those hard corners
catch tank - stop the fuel surge through hard corners
air flow - air filter, power rohr, inlet manifold, exhaust manifold, exhaust system and of course there's the big valve heads, ported, polished etc etc
LSD - slip diff for an extra tug out of the bends, MUST HAVE!!
