Springs and shocks make the largest single difference in a car as far as overall suspension performance.

Springs: 88 SS springs are somewhat hard to find compared to other parts for your 300ZX. To begin with they only made 1000 "Shiro Special" 300ZX's in the United States. There was a limited availability of the same springs from Nissan, but the supply ran out long ago. I chose these for two reasons; they are the stiffest "drop in" spring and they keep stock ride height. Both of which I wanted. Suspension Techniques makes springs with VERY similar rates, howeverr they lower your car to the point where you need to correct the rear camber. I previously had Eibach springs, which were a fair change from stock and offer good ride quality.

Struts and shocks: If you don't know what they are, you should not be working on a car. The only real option for the Z31 in terms of a quality, inexpensive adjustable shock is the Tokico Illumina. Koni's are available, though much more expensive, it can be argued which are actually better. KYB's AGX's are not very good shocks, and should not be considered "performance" parts at all. They are probably alright for a regular street car. Without a doubt the illuminas offer the best price/performance in any adjustable shock you can buy. Illuminas for the non-turbo cars have been discontinued since 2000, good luck finding them.

That said, if you have a non-turbo car you can swap in the turbo model's strut tubes from the appropriate year for your car (84-86 or 87-89). These strut tubes can usually be had for roughly $50-80.

Strut cartridges and springs are fairly easy to install yourself with basic tools and a spring compressor. I am not going to explain the process, since it's simple AND in the Haynes manual (which you should already have). Here is a picture to help people that have never seen a strut.