It seems that although the Mk2 is lighter and can be easier/cheaper to mod, good shells are getting harder to find and more expensive. So as DH said a Mk3 is probably the next best bet. If you haven't done track days, starting out with a Mk3 16v GTI will have enough power to have fun without being stupidly expensive and parts are easy to find. (Avoid the VR6 as the engine makes it rather more nose heavy). It'll have better brakes and safety kit to start with than a Mk2. Strip it out to lose as much weight as possible, better suspension, new tyres and so on.
Once you're got a taste for it, then you can up the power, improve the brakes, cage, harnesses, etc.
Take a look through the Dub Rides and Projects sections, there are a few track cars in there for inspiration.
