If you are building up a 5E you will need a new clutc, it uses a smaller 200mm clutch. And no, you cant fit the 4E flywheel to it which sucks.
As for power, plenty of people are running 250bhp+ and never seem to have a problem. Depends how you treat it i guess. It wont last forever (my gbox hax done 126k miles or there about on stock power, ad its near the end of its life (2nd gear syncro fooked and 3rd gear is hard to engage)