That's only needed if you want to make sure it's all good, and safe, and reliable, and runs etc.
I reckon I could do it for farrrrrr less than these guys are saying.
Fab up a dodgy pipe to go from the twin exhaust outlets to one, a turbo from Bunnings for $400, then fab up a new outlet pipe.
I reckon I could do a high mount turbo for under $2k... Now whether the motor grenades when it comes onto boost the first time... That's a different question. And I'd hope, being a Skyline, especially a GTR, there is more put into it than a dodgy Bunnings turbo...