Depends on what kind of track car you want to tow, and the size of the trailer. If it's a big car and/or a big trailer, you're going to need something with decent toming capacity which possibly rules out a fair bit of stuff. On the other hand if it's light then anything will do.
Personally I'd want something wafty and expendable, so something I don't mind chucking tools and tyres into the back of when it's cold and wet and I just want to get home. Pretty much that's going to be a pickup or a leggy estate. I've enjoyed all the Ford Rangers I've had, and the current one is pretty comfy and better specced than the rivals (when new anyway, no idea on the second hand market). Plus the Yanks are desperate for Ford to release it over there purely because of the Ranger's massive towing ability, and they don't tend to mucj around with towing small things
E61 535d would be a great car for towing stuff, I've seen a fair few people at the circuit with them. The equivalent E-class might be even better, certainly for wafting.