4x4 Cossie brakes should fit under your 15's although some wheels won't allow them to go in. You may also need to balance the wheel slightly differently to give clearance.

Leave the rears alone as they are quite up to the job at hand. They look a little agricultural but they do work. Why change them for something more aesthetically pleasing that tends to cause more problems than they cure.

A good mate of mine still runs stock drums on the back of his 400bhp ZT monster FRST