For power you have so many routes to take, so i'll just highlight a few things.
If you know a lot, you can do this work yourself, but if not, get a reputable shop to do the engine building for you. Yes it's expensive but when i bought this car i was pretty new and i've never built an engine before.
I've done some minor work myself like changing over the rear diff, but if it's stupid complex you really will want some quality help so you don't fuck shit up, if you don't 100% know what you're doing.
The other advantage of a reputable shop is they all own their own GTR's and have years of development working on them. So you get access to their own propriety technology, which can be pretty cool.
A good resource for checking out what's good is the youtube channel motive DVD, they have a few good videos on things such as turbos, ect
There's also SAU (the forum) which is full of nice people who also own GTR's and can give you advice. If you're in doubt, ask, it's worth getting it right the first time.
I actually got my electric mate to do my immobiliser, so not sure. He's done a full kill switch where it needs my keyfob in the car to turn on, it's some crazy system. After the highway incident i was really paranoid about it getting stolen.
It's a very expensive car, though it does not seem so on the surface. I've spent well over 100 grand so far, but i don't regret it for a second.
Just remember, a built GTR is a supercar, it comes with the maintenance and it's pretty complex on a few little things (eg: screwing the diff in is actually quite fucking tricky).
Make sure you're budgeting for shit going wrong, because the last owner 100% has thrashed it to hell, no matter what he tells you. That's ok though, cause you'll be replacing wear items.
Just make sure no rust, if it has rust avoid like the plague.