Don't know what the series is and too lazy to look it up, I assume is similar to the Lemons Series races near me, which are a lot of fun.
Make sure your harness is up to date, and if you get a used one that it was not in any accidents. The harnesses will stretch in accidents causing them to have weak sections afterwards. I think all regulations require the harnesses to be replaced after accidents. I believe the life span for the up to date-ness of a harness is about 2 years, depending on the certification ratings. Like Dylan said, the 4 point harnesses can cause spinal compression and submarining, so keep it 5-6 point.
A full roll cage will make your car unsafe to drive on the streets without the use of a helmet. If you plan on driving the car after the race, make sure the roll cage is either removed or modified to keep your injury risk down. A half roll cage or roll bar is safer for street use since the bars are not next to or in front of your head, however the bars will still be behind your head and can still cause injury.
Bring a rule book with you to the shop doing the work, and have them log the work that is being done on the car. Some organizations require log books detailing service and modifications along with the dates and certifications/ratings for the car to be eligible for competition.