Offset depends on the car. The more you stray from what the hubs were designed for, the more wear you put on the wheel bearings... which can be bad on a race track. I'd stay within 10-15mm of the factory offset unless you want to plan on replacing wheel bearings regularly. The benefit of a lower offset wheel is wider track, which can improve handling... but don't go overboard with it. Unless you're hacking fenders, fender clearance is a limiting factor, as well.

Otherwise, you want width that is appropriate for the tires that you intend to use, and the strongest and lightest wheels you can afford.