I'll give a little bit of advice...
The rear brake swap and front brake swap are equally easy.
The part that makes the rear brake swap more difficult is utilizing the e-brake. If you feel that an e-brake isn't too important, you can swap in the rear brakes just as easily as the fronts. If you're getting both front and rear z-brakes for $200, you definitely do not want to pass that up. It's a steal.
And I can't remember the necessary tools off of the top of my head, but I'm pretty sure the caliper bolts are 17mm... and the caliper/brake line nut is 11mm(not sure on that one). The brake line/hard line nut looks like 13-14mm or so... but everything can be done with ordinary hand tools.
Also, speedbleeders rock!
And most importantly(if you didn't know before) Most wheels will not clear the 300zx front calipers(30mm). There are certain s13 alloy wheels that will clear the 26mm ones, but I'm not sure how to tell which ones will clear. s13 alloys will clear he rear z-brakes.
good luck.