$ 300.00 worth eh....tough but I guess you have to base your decision on what is the top priority for you.
Titanium ball cups
K-Factory side brace(helps protect fuel tank)
Replace with a thicker lightened racing chassis
Titanium Pillow Balls
Titanium or Carbon fiber radio plate.
Better exhaust system
Better steering servo(digital is more linear)--won't make you
faster but the car will definitely respond better and faster
In my opinion, the best upgrades for your $ 300.00 buy another kit, then you have all the spares you will need because even with upgraded parts they can still break. This is the most cost effective way of getting spares for your car.