I would recommend keeping the car as is and run ST. You have the majority of ST mods already. You can find a helmet for autox for under a $100 with no problem.
Will you be the fastest ST car? Maybe not, but that way you can keep your car as is. Go race, have fun! Save some pennies for a good set of tires for ST.
Work on your driving skills, once you are getting the most out of the car, throw on some konis.
Parking Lot Hero
2008 WRX daily driver
2003 ZX3 23D Autox Car 42 FSP
"Hella Functional - Raced, Not Hard Parked"