Having a decent seat and harness will help immensely when going fast.
I daily drove my Targa car with a full cage and race seats for a couple of years, was not so bad.. I would retain a simple lap/slash belt if you can though, nothing more annoying than getting all harnessed in and forgetting something and having to un-do it then buckle it all back up.. even leaving your sunglasses on the dash requires an unstrap of the harness to reach them, it can be annoying.
Best of luck. There are plenty of good authority card inspectors out there and if you enter a couple of club-sport events a year and have the log book stamped you won't need to have the car inspected yearly.
My first port of call would be joining a car club, from there find out about events and local inspectors etc. Can recommend Pukekohe car club, they are quite active and have some great events.
Getting a Clubsport licence is easy also.
Oh, one down side is that only people with motorsport licences can drive your car, so you can't have your mates drive it, or the missus etc... I got my girlfriend to get a National Rally licence, told her she could have a drive of my car.. Then she realised that my fixed race seat meant she can't reach the pedals. Win win.