I see your point alright, much more hassle throwing in the 6 speed so put it like this..
Option 1 - 5 speed from a classic Sti and R160 rear diff to match (No messing, straight in)
Option 2 - 6 speed from newage Sti and R160 to match and also the sti prop shaft from a newage model (prop is £100 in breakers up north), gear linkage, clutch, flywheel and starter also required.
Option 3 - Basically option 2 and everything on the rear beam
Lots of Classic Sti boxes around with the amount of people doing 6 speed conversions, would be a good option if your box was knackered but i wouldnt bother otherwise. WRXIan is running over 300BHP and unless I'm mistaken, he still has a newage WRX gearbox in his. If the upgrade is intended to handle bigger power then, I think the classic 5 speeds can handle around 400, hopefully someone can clarify and say for definite. The 6 speed is practically bulletproof and have been proven to handle up around 500BHP. What type of power do you intend to run eventually?