Looks like an US spec car to me from the bumpers and side indicators. A very brief breakdown of the models is:-
Lux was the 2.5ltr 8v car with circa 160bhp. I'm not 100% up on their exact spec and performance figures, but i'm sure others will list more details.
S2 has a 3.0ltr 16v engine with 211bhp. Has uprated brakes and suspension over the lux (4-pot Brembo's all round). Has a 'wrap around' integral front bumper/nose as opposed to the Lux's two peice bumper and under-bumper valance.
Turbo's had had 2.5ltr 8v turbocharged engines. The early models ('85 - late '87) had 220bhp and around 6s 0-60, and the same brakes and suspension as the S2. The later models had 250bhp and 0-60 in around 5.5s. Later turbos also had LSD and uprated suspension and larger 4-pot Brembos up front pinched off the 928S4. Turbo's also had the same one-piece front bumper as the S2.
For a track-toy depends what you want to do. If you want something to track out of the box then you're probably better off with a Lux or S2. If you fancy a bit of modification to get more power then the turbo is your best option - but be prepared to spend some bucks.
968's are prodominantly an S2 with the obvious cosmetic mods. The only significant mechanical difference is a 6-speed box and the 3.0ltr engine got a cylinder head with Vario-cam - i.e. variable valve timing (not variable lift though). I think these were 240bhp off the production line. If your budget stretches to a 968 and you want out of the box track car this could also be a good choice, but for the same money you could get a decent S2 and spend the difference on uprated suspension, brakes, ECU chip and a few other track orientated bits and bobs and you'd end up with a better track car than a standard 968.
Ultimately depends on your budget. Either way for a track day car you can do far far worse than a 944. They are very balanced cars and very rewarding to drive. Also they are more than capable of being driven to the track in a good level of comfort, being driven round the track all day and being driven back from the track.
Hope this helps to set the scene.