Dittos on everything Trip said. Pungo is a good place for your first event. It is very low key, the course is technical, which means you have a good chance of learning quite a bit, and the speeds are fairly low so you will not get in over your head.

$$$'s spent on entry fees are more important than $$$'s spent on parts when it comes to making your car go fast. By you definition, they might be 2 or 3 cars that are "done" at any autocross.