I cant help you with all the Questions but If i was you i would go the OS Gear set 1-5 + upgrade imput shaft...
I got a few mates using these gearsets with about 50kws less then what your making...
But from experience There Very smooth to drive and u wont have to worry about breaking it ...
Best option for you is to call around a few gearbox places that have done these box's and know all about them, then ask them for pricing in regards to extra parts needed etc...
I think alot of sau members use award diff and gearbox.
Hope this helps in some way