well it's the same options you have with any g40, all the same stuff fits a mk2.
Depends how much you want to spend:
< £200 - new pads and vented discs with VWII calipers (basically standard g40 spec). New rear shoes, new brake flexi hoses
£200+ - 256mm audi calipers and carriers, braided flexi hoses, new rear shoes & drums
£500+ - wilwood 4-pots, 256mm discs, decent pads, braided flexi hoses, new shoes and drums.
Don't overlook the rear drums, even if you're not upgrading them (which to be honest, isn't needed) it's worth refreshing them. If the rear brakes aren't working well then it'll affect the whole braking of the car. I upgraded mine to 200mm using parts from a passat. To be honest it cost the same as buying new 180mm parts so was no big deal but I doubt it makes any difference really. The rear brakes do very little as there is no weight there, esp with a stripped out track car!
There are options to fit a servo but either involve cutting & welding the builkhead, fabricating some parts to fit a small servo off something (people have used metro servos in the past) or making a remote servo kit (which you can either buy as a kit or get from a mk1 golf). All a servo gives you is assistance in pressing the pedal it will not make your brakes any better themselves just means it requires less leg effort to operate them.