Yeh a good start is to replace the front disc set up with S2 calipers and discs, also replace the old master cylinder with the S2 one
As for brake linkage you could start by replacing the bushes, this'll make the brake travel shorter and feel alot better
I have the standard set up on mine but with mintex pads and grooved disks new bushes in the linkage and new shoes and drums at the back all nicely adjusted and i find the braking is excellent of you keep them maintained
For a Turbo conversion its probrably easier to get a rotten/written off RS Turbo and swap the whole lot over, loom an all, not one for the purists mind you
Im fitting a Turbo Technics conversion to mine, which keeps all the original injection system/loom etc including the twin coils
mind you the Turbo conversions arent easy to get hold of nowadays