Another way of looking at this topic is to consider the "value for money" equation. In this regard, the z31 is pretty good value for money for a zed car if you get a good example. You have to way up what's important to you. Compared to other makes and models you may well have a different view but if you are particularly interested in zed cars only, then z31 is one of the more affordable models.
If you are looking for attributes such as reliability, economy, safety and modern conveniences, you could probably recommend that a young lady look at something near new. For the price of around 14k you could probably get a 2008 Lancer from Manheim Fowles Auctions. If you want something larger a 2008 Mitsubishi 380 is good buying as well at 13k. Any young person breaking down in the middle of night on a busy street is not a pleasant situation to be in.
One of the other things I've thought about since my previous post and others is the issue of maintenance costs. Lately as I've been looking at 260z's and 280zx's I have found it very hard to find an s30 without a rust problems and even then the asking price is above 10k. Then when you compare the 280zx to the z31, the z31 feels much more modern and safe inside. While the 300zx could be costly to maintain mechanically the older zeds will be more costly when you decide to cut out rust. I've gone to see quite a few and there are many shiny cars freshly repainted hiding rust to varying degrees. A good magnet is handy.Some rust you can live with, others are a structural worry.
The other thing in the z31's favour is that when they were new, they were pretty expensive, were bought by well to do individuals who pampered these cars and being less sporty than a z32, rx7 or supra were maybe gently driven. The trick is to find one that has been looked after like this, and yes, they are out there, you just have to been patient because they are now 28 years old (1984).
Hope this helps, and good luck in finding a car.