Don't put any mods on your car. Run it as is.

Cause I can bet you $100 that I can beat you in your stock car and then you put mods on it. Since you are a novice it's best to learn the limits of your car first and see what class is appropriate for you than to waste money only to find out you are in the hardest class.

But yes all autocross courses are limited to highway speeds (around 65mph). A tour was protested this year because speeds reached in excess of 80mph I believe.