Paul S covers a lot, but I'm going to write my inteligence of the 924, 944, and 928.
First of all, I own a 924. I like it very much. It is definitely not the fastest car GM Tech 2 on the planet, but where it lacks it makes up for. Its handling is exceptional and some say it is the best handling of all Porsches. Many parts are Audi and VW, but the top half of the engine (the head) is Porsche and the torsion bars in the back are porsche also. If you are going to buy a 924 that is a 86 to 88, then you'd get the 924 S, Which has the original 944 engine in it. They are wonderful cars that are faster than the early 944s.
The 944 was built by Audi also, making the 944 just as much an Audi/VW but it contains a completely Porsche engine. It handles much like the 924, except it is heavier. They changed the interior in 85, which was a step up from the old 924 based interior. Faster than a 924 though.
Now the 928 is a GT compared to the 924 and 944 which are sports cars. The 928 was made by Porsche, which makes it the most "genuine" porsche (they're all Porsches though). It also has Porsches first and powerful V-8. The ones produced around 86-88 were producing 300 to 330 hp. Interior was nice also.
The 928 is the best, but pricewise you can get a very nice porsche 924 with power Tech 2 sunroof, power mirrors, and other power options at a bargain of a price.