It doesn't sound like a brake upgrade would make much sense for you right now.
There is nothing wrong with the stock Z brakes for street driving, especially on a 240Z. Get some rebuilt calipers, new pads, rotors, etc. and you should be all set.
That setup would need wheel spacers in front and 15" rims in back. Some people have gotten Z31 discs to fit under 14" wheels, but I'm not sure how.