I've installed, heard, bought, sold, etc all different types of exhausts and generally with eBay stuff you're going to get lousy fitment and the resonators (if it even has them) and mufflers are going to need replacement MUCH faster than a higher quality system. There are exceptions, but I'm not aware of any for the Z34.
One way to save money would be to buy just mufflers and have a shop fab you up some piping. If you can find a good price on labor, you'll save a bit of cash. I did this on 2 of my cars, and was very happy with the results but I had access to tube benders, welding equipment, a lift, and a friend that knew how to weld pretty good. Here's a few mufflers:
HKS Mufflers
Your next best bet would probably be picking up a used exhaust from a reputable member on the forums.