Hey Prelude,
Given it will be your first time. You will select Novice under PAX and HS for Class. I believe the two modifications still fall under HS. If you are running a wider wheel that will bump you into a new class. I point this out as it's one of the most common changes people make.
As for Time Only's. Those are "Fun Runs" After competing in the morning groups you will then have Groups E or F that is for getting additional runs in the day. Great for continuing to improve as a driver.