The rollbar extension is an excellent upgrade even if you don't track or autocross.
Improves safety and performance.
A guy in one of my clubs offered me one for sale when he moronically went from BoxsterS to M3-- well at least he ditched the benchmark roadster for the coupe and not the drop top M3.
Yeah no need to drop more than a couple hundred on a helmet for atuocross.
I have a 2nd hand nearly brand new Bieffe F1 from Ebay.
If anything my helmet is overkill, a rally style helmet with the open face is a better option because it get bloody hot waiting for you lap in the middle of July.
What I might consider is a set of harness belts if you can fit them to your seats.
Some run them under the stock seats. The belts make a big difference on lap times as you take weight off the steering wheel, you're in good.
They also sell the inexpensive CG lock to keep you anchored at the belt a good alternative if you don't want to spend hundreds on a six point
http://www.cg-lock.com/