For insurance try Famous Insurance, their website is
http://www.famousinsurance.com.au/ or call them on 1300 88 60 29
I don't know how they would be with a N/A > Turbo conversion but they were great with my Skyline. It was a R33 with all the usual stuff, mags, front mount, high-flowed turbs, Apexi Power FC, etc... I was 22 at the time and it was insured for 20k, my premium was 1600... sounds like alot but that was insured in my name with all mods listed, other companies were around the 5k mark. May be cheaper if you only want to cover 12-15k.
Good luck with the turbo conversion if you decide to go ahead with it.. I drive an AUII Forte as well and I can see why you want the turbo kit. And as for being so young for a turbo, I wouldn't worry about it. When you've spent a fair amount of cash on something, plus time and effort you tend to look after it, that would probably mean not going crazy on public streets. My first car was a 13B turbo RX-3 that I built with the help of my dad... it was an animal of a car and probably the quickest car I've been in.. I learnt to drive on my L's in it as well.. Because we used to spend almost every weekend under the car, and everything I earnt from working s**t jobs at $6 per/hour was forked out into the conversion, I had much more respect for the car... Probably the reason I never wrapped it around a pole.
Anyway, enough ranting from me.. Good luck with the conversion if you decide to go ahead with it. Would be great to see a turbo forte.