Yes, genuine N1 turbos would be pricey at the same time rare to find.
Garrett -7's would be the closest to the N1 equivalents.
I am running Garrett -7's with 15.5 Psi, which is just over 1 Bar.
Later I will run about 18 Psi = 1.2 Bar, but will get my mechanic to see what he thinks best.
Many factors are involved but you'd run what's safe for the engine / what the turbos can handle, so when you go in for a tune after the twin turbo installation, your mechanic should be able to gauge what it can run.
Not that I know of... That theory might apply to N/A cars... As you can only go so big unless it's tuned for it...
With turbocharged cars, usually the bigger the better, but 3" / 3.5" is usually the upgrade we all go for and it frees up the exhaust flow.