S14a is the facelifted (and most think the nicest) version of the S14. They have a more aggressive looking front end. S14, S14a and S15 all have the SR20DET engine (some were SR20DE, which is non-turbo, but not many over here).
They are great cars for the money, providing, of course, you get a good one.
Expect VVT rattle, which will cost around £500-600 to sort, but it could be argued that the root cause is a high mileage or poorly maintained engine. Big-ends are another common cause of problems and can easily begin to clatter at 80,000 miles.
Watch out for Jap import cars that will most probably feature parts that cannot be worked on over here, like certain ECU's and Coilovers (which may need rebuilding, but may not be supported in the UK for parts). Also, imported cars usually have a cloud of doubt over the detailed spec, which can be restrictive.
The SR20DET has a timing chain, so no belt to replace. Rear bed bushes are another prone area, there are 4 bushes, 2 of which are silicone filled. These become sloppy and are a fairly big job to replace, although there are other options.
Anything else??!!!