Shannons will insure anyone. The under 25 thing is bullshit. I got my Modified GTR insured through shannons when i was 21. Basically the same as Richard. Forged motor, -7's etc. had no issue getting it insured and i did it all over the phone.
IF you get bored of doing your car you can spend some time on mine Loving your work. Made it look a shit ton easier than it was. My mind is blown at the attention to detail for the engine bay. Something im only just starting to work on.
For a starting track car simple Coilovers are simple and cheap. all the discussions beforehand have been about the engine hence the input.
It is not hard nor costly to make a GTR handle beautifully.
9 Months for a ground up rebuild is normal TBH. Mine was in the shop for about that. Im still going too. Only little things to go. The only part that is original is the radiator ATM. getting replaced next month. Everything else is brand new pretty much.
I wouldn't risk buying a GTR without putting it into a shop before you track it. Or buy one cheapish and track it till it does break. If you are willing to put the time in (time more than money) they are worth it.
Other option is to possibly find a GTS-4 skyline and do a 30/25 conversion into it. Just an idea. (cheaper!)
Evos typically cost more. You can usually buy a GTR and get it built to how you want it for the price of a good evo....
In saying that:
My mate has an evo 9.
Exhaust+ up the boost see 220KW. Job done.
Edit: in saying that i did notice Op saying 5/6/7 evo. I would get a 32 to be perfectly honest. Though i am biased.They are cheaper and quite a bit lighter than the other dielines. And it doesn't cost that much to get them "Safe".
Strip them and they really come out of their shells. (so to speak)