Good choice on the helmet, but I think you'll have a hard time finding the SA2000 version. Soloracer only has smalls left on their clearance shelf.
I use my harnesses on the track as well. I figure the increased risk of injury in a rollover is more than balanced by the better feel and control I have of the car with harnesses on. I have a 4-point g-Force harness in my miata that will be a full 6 point harness once I put in the race bucket. And in a collision, I would much prefer to be wearing my harness than the stock belts, since in my case, the lapbelt is much higher than my hips for some reason. If I hit something head-on I'm gonna have some internal injuries with the OEM belt on. :-|