Couple of things.
First you need to determine class you plan to run. Stock class doesn't allow much in way of mods other than tires and front sway bar.
There are some things you can do with your setup now that will help eliminate understeer as well. Do you know what your alignment specs are currently? Put a little toe out in front, put some negative camber into the car as well.. What tires do you have and what pressures did you run?
On oil cooler. Unfortunately oil cooler bumps you out of stock class but at local level I don't think anyone will protest you if you run stock class. Oil cooler bumps you to BSP. A good oil cooler will keep you around 220 at ax events and 180 to 210 on street usually.
Brakes. Just put a good pad on the car. You don't need muchnin the way of brakes in AX. Replace brake fluid with high quality race fluid like Motul.
If you want to go beyond stock or just run for fun I would put good set of shocks and springs on the car. Good swaybars and upgrade to a top notch street tire like Bridgestone re11, Yokohama Advan a008, etc.
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Grant
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