The 1.7's are great motors, although at high BHP the VVT becomes redundant, plus you're only realistically looking at 10bhp at the top end over a 1.6.

Simple breathing mods really help them rev, I'm only running a Green cone on my Puma and it revs fantastically.

As for rear discs, Focus rear hubs are the done thing, but make sure you have hefty brakes at the front else you'll be over braking the rear with discs.