I agree on the 'wait til you've got it' front. Have you driven this kind of car before? 140-150rwkw might be enough for you for a while.
Anyway, going by previous car purchases (performance cars OR not): Allow 5-10% of the cost of the car in repairs in the first 2 years. Something WILL break, regardless whether you mod it or not. That's been my experience so far.
Then WRT mods: You'd be looking at another 3-4K or more in labour if you're having all that stuff done. It may be because I'm at work and none of the pics you've added are coming up, but I don't see anything about suspension or clutch. Add another 2-4K depending how far you go with them.
By the way, before you apply for a 35K loan (like I did), please be aware that most of the major banks have 30K as a glass ceiling that people without "assets" can borrow. And by assets, they mean if you don't own land, or a (resaleable) car, or stocks or anything, and you don't have a LOT of disposable income, it's HIGHLY unlikely they'll lend you over 30K. And rejected loan applications from one institution impact on applications to other places. Just sayin', cos that's what I found and ended up going through a (knee) broker ( ), you might be in a different situation.
Because 3 years ago, I had the same wonderful dreams you currently have: My R33 GTST (so same engine as what you'll be getting):
Car: 25k (bit more than should have paid, but was close and in pretty good nick)
Instantly needed new coilpacks (fault was hidden by the seller, only figured it out a few months later), 500
Water pump and radiator were knackered after my first really 'spirited' drive, few hundred
Front rotors had signs of cracking after 1 year, 500
About a year ago the suspension was found to be on the front bump stops + other issues, 2K
And none of this was power upgrades or appearances etc (which I have also done, but they add up to way more than the boring 'fixes').
Get the best car you can afford to, keeping 10% money spare. If nothing falls off/blows up in the first year, THEN think about upgrading it. Just bare in mind that upgrading one thing often breaks other bits.
Ever thought of just buying an R33 GTR? If you look hard, you *might* find one for 35-40K You'd have a bit more power than a 25DET, and the GTRs are built a bit stronger. Plus, you hopefully wouldn't feel the urge to upgrade it for the first year (if you do.... eyes bigger than your stomach ).