Just work on tuning what you have got. Work hard on driving technique. I think you can be competative stock with a nicely setup car. Driving is more important than the car. Just learn your car really well. If you see yourself outclassed on the track in certain areas, focus on how to reduce the edge or upgrade a few parts to make a difference in that area. It is hard to just say upgrade this and that
. Figure out what you need to upgrade first. It will take lots of testing, but it will save you lots of money and pay off much more in the end.