To the best of my knowledge (some conditions may apply)...
coilover suspension - illegal (only if you get caught) :roll:
race seats - legal if they are ADR approved.
momo steering wheel - Legal as long as its not too small (don't know exact requirements)
either 17s or maybe keep the stock 15s - Legal generally as long as the the overall circumference isn't different to stock by more than 15mm.
bigger turbo - touchy subject. may have emissions issues.
bigger intercooler painted black,no cut bumper - legal.
blow off valve plumbed back in - legal
air filter in air box - mostly legal, but some transport inspectors can be idiots.
have all engine mods look as factory as possible - wise move
possibly a half cage for safety during events - legal if you have a mod plate for it and the car is changed to a 2 seater.
Basically, once you have bought the car, there are no inspections required. BUT, the transport department do have road side inspections where they pull you up and go over things. Also, Cops can pull you up for things and force you to take it for an inspection once it's fixed.
Basically though, if you are not being a goose on the street, then you will very rarely have a problem.
Whether your mods will be covered by Insurance is up to the insurer.