Neil i love my R33 it is a lot of fun and holds some sentimental value with me as does yours with you, you will get great satisfaction out of the transformation i know i did with mine.
My car only has 250-270 rwkw (depending on boost) but is sure does come on hard, last time you were in the car it had rubbish tyres with good rubber and a little time spent on the rear and set up you will not have any dramas with traction from 3rd and up.
The parts you have chosen look great, how much adjustment do you get out of the Nismo arms? All my arms are std with adj whiteline bushes they are ok but i want to solid mt the diff and cradle and run rose joints through the rear end. As for the front i have purchased a Chinese set of adj lower control arms and guess what they DONT fit woohoo so now i am looking for another option. I will be interested to see how much adjustment you can get out of the arms you have.
The std motors will last a long time if looked after correctly, you will have no drams here mate.
As for the cage they provide a great sense of security and really tie the car together, after driving mine with and without a cage you can really tell the difference. Just make sure it is designed specifically for YOU, it is hard letting someone cut your pride and joy up but if it is done write it will be very rewarding. Just make sure if you are getting a dash bar that the factory dash and cluster fits before it is all finished as mine didnt, if you are anything like me stuff like that is hard to fit into the budget.
My phone is always on if you need have any questions, i will be following your progress.
Good luck,
Ryan