Here are some answers to the common problems we've found with the 300zx. I plan to do one about 240-280Z's soon.
The main problems we see with the early 300's are:
1) Bad air regulators causing low idle speed when cold apox$125
2) Injectors leaking, dealer are replacing for free as a recall but this is also a problem in that mechanics are not getting paid well by nissan to do this and we have seen some shoddy installs because of this and they won't accept responsibility.
3) Air flow meters are very easily damaged from dirt and cars will run very poorly. Most common cause for running problems. Nissan dealer air filters are a must. A new meter is aprox$550, python rebuilds are about $250.
4) Cam belts and tensioners need to be changed every 60K as valves crash hard on these engines and cause alot of damage.
5) Electric adjust shock are very expensive to replace and dealer shock are only ones that fit and work right $1200.
6) These cars get trans mounts that grown when accel. and when they get replaced can accentuate drive shaft vibrations.
7) spark plugs are VERY hard to change-stupid design here.
Other than this, just the normal leaks ect. that other cars develop and the engines are good for 200,000+ miles. Nice car for the money. If you ever think you will want performance upgrade don't get non- turbo as nothing can be done to improve the non-t cars. The turbo engines are just as reliable as non-turbos and see very few turbo problems..enjoy...
 On the 90 and up cars here is what we've seen:
1) For some reason the 2+2's have probs with driveshaft vibrations the coupes don't have, fix is new driveshaft $750
2)Turbo models have problems with oil line on right turbo clogging and ruining turbo. Nissan has updated line with insulation on it to keep this from reoccurring. We recommend replacing on affected models before turbo goes out.
3) Ign module recall-update. the oe module has problems and nissan replaces them for free under warranty.
4) Camshaft adjuster noise-leaks. These cars have hyd. cam timing adjusters that rattle and leak nissan has new version but this ones on you about $400 apiece + labor.
5) Auto trans models have problem with planetary gears exploding and ruining trans and radiator especially turbo versions. We don't recommend turbo automatics to our customers.
The engines seem pretty durable (alot better than turbo RX-7's!) but they use some oil, must be checked regularly. We have seen several people run them out of oil and ruin the engine. It's very expensive, and alot of labor, to replace the engine even with used piece, especially the turbo models.
Well this is an overview of the common problems we've seen.