I think You need to consider what your life and quality of life is worth vs. cost of safety and risk incurred.
You should consider which tracks you will use. Some are flat with lots of run-off and curves to keep speeds reasonable and incidents less traumatic.
You should consider people that you track with. There are all kinds!
HAving a heavy 3500lbs monster with 700hp run into you in a Lotus would be highly unpleasant.
I would then take an SCCA rule book and try to apply as much of it as possible.
1. Rear toe links have to be fixed/replaced.
2. All suspension and brakes should be checked before every event.
3. There is a rear hoop built-in. It is of good size. It needs belt bar and cross-bar. Putting something in will not detract from the car at all.
4. The roll protection over the winshield is not good. It is just a flimsy fiberglass frame. I am not sure how to fix the knee bar, an equivalent of front hoop and side bars.
5. Side bars inside doors are OK, but not regulation. If that 3500lbs monster t-bones you, you're gone. This happened to a Trans-AM car at Mosport at 160mph. The driver died. Trans-am and SCCA regs where changed for better side intrusion.
6. Real fire suystem ($500), not the clip on extinguisher is a must. See posts around here about fires.... (Clip-on extuinguishers mimick NHRA rule. Remember drag racers carry only 1gal of fuel, you have 15! It burns long and hot!).
7. 5 or 6-point belts can be installed with the regualr ones for track use. I used to run stock belt and 6-point in my Vette. Used the stock on the street and 6-point on the track. This is a $150 expense.
8. Buy proper helment, suit, shoes, gloves and baclava. Practice geting out of the car quickly.
9. Keep roof on and windows at least 2/3rd up to keep you arms and head in the car.
Upgrading wheels, tires, brakes, sway bars, bushings (to spherical) will add a margin of safety, as well, as making your car more enjoyable on the street.
You maybe able to get HANS to work with aftermarket seat....? Has anyone tried?
I see side intrusion and lack of front roll-over protection as the biggest issue that does not have a simple solution.
EXCELLENT write up! Anton hit on key points. Toe links and inspections are a must. Adding a harness bar and multi*point belts are a must and do not take anything away from a street car. Remember that you are talking about your life here. Small expenses to keep you alive. Enough said!