There are rather a lot of variants, designed to fill every niche and get the maximum number of sales out of one platform for Porsche.
The basic car is a Carrera 2 (C2). This is rear wheel drive coupe and was the first variant to be produced. With 3.4 litres and 300 bhp from '98-'02, then 3.6 litres and 320 bhp till it was replaced by the 997 last autumn.
Then there is the Carrera 4 (C4). This is 4-wheel drive, but otherwise the same as the C2. The 4-wheel drive makes it feel a bit more planted, but it adds weight. You also get less space in the front boot, since the front diff takes up space. Available in 300 and 320 bhp form.
There is a wide body version of the C4, called the C4S. This has the wider wings, 10mm lower suspension and brakes from the turbo. However, it doesnot have any more power. These have been very popular, and C4 sales basically dried up when it came out. It was available for about 2 years, so you won't find any over 3 years old. All have 320 bhp.
All the above can be had in cabriolet form. This adds weight and makes the car less stiff. The roof is soft, although there is a hard top you can put on in the winter if you want.
There is a targa, which has a big moon roof and a back window that opens, making it a but more practical. It is based on the C2, so there is no 4-wd version. I think this came out post facelift, so only available in 320 bhp form.
For all the above cars, there was a factory engine upgrade, which cost about 5 grand I think, that put the power up to 345 bhp. Not many cars had this.
The GT3 has had 2 production runs. The first was quite limited. For the second the cars was built more-or-less to meet demand. The Mk1 has 360 bhp, the Mk2 380bhp.
The mk2 run has more power, but more weight. Performance is about equal I think.
There was a very limited run of the GT3-RS. This was a homologation exercise - Porsche needed to sell a certain number to get some parts qualified for racing usage.
These are mostly for track use, although you can use them on the road. The ride is hard. They come with proper racing bucket seats, which are very supportive bnot so practical for the trip to Sainsbury's.
In racing trim, the GT3-R, GT3-RS and GT3-RSR have been very effective.
The racing versions can only be purchased through Reading and payment is to the factory in Euros. They are very limited and not road legal.
The Turbo has 420 bhp and 560Nm of torque.
There is a factory engine upgrade which a small amount (maybe 5%?) of cars have (option X50) that gives you 450 bhp and 620 Nm.
This is pretty much classed as a supercar, rather than a sports car, like the normally aspirated versions. It is considered by the mags to be one of the best cars in the world.
You get stability control, which does not intervene much, will let you spin the wheels and even slide it a bit.
It's pretty quick and is only available as 4-wd.
It can be endlessly tinkered with (if you have a big enough pile of cash) to give up to 850bhp. As stanard it will do over 190mph, but once played with 210 is quite easy, and 230-240 is possible (but the mods cost as much as the car).
The GT2 had its power upgraded about 18 months ago. It was 460ish, but is now around 480ish.
This is a turbo with a bit more power, rear wheel drive, no electronic stability stuff, racing suspension (no rubber bits), bucket seats and is quite focussed. It's not as a hardcore as the 993 GT2 was, since it is not the basis for racing.
I have read that it is not so easy to drive flat out, and a GT3 will get a similar lap time (and cost you a lot less).
Sales of these are small, 20ish per year.
If you want a no-nonsense value for money version, then a standard C2 is a pretty good choice.
The C4S handles very well, and was Evo's car of the year a few years ago. Some people prefer rear wheel drive, but I think the C4S is a very good car.
The GT3 is the best track car, but is less practical for everyday use.
If you are used to turbo power, I would suggest the Turbo is the best fit if coming from the Merc.
The best thing to do is drive a C2 and a Turbo, then see what you think.
