Considering your low mounting the turbo, I don't think many people will be able to help you. Im going to take an educated guess and say, yes, you will need to modify the subframe. Do up some basic measurements and see where you get to.
I personally think you should reconsider the low mount idea, but if thats what your hearts set on then go for it - there just isnt much info out there to help you, and even less (probably none) for a RHD. You might have to be one of the first. One of the real 2002 Turbos going around is a RHD - owner claims factory but I'd be more inclined to believe the factory figures.
A GT25 will be fine if you aren't chasing big hp.