Run STS. When you wear out the ES100, replace them with Kuhmo MXs or Falken Azenis autocross tires, or BFG g-Force KD (not the KDW2) - if you can afford them (I can't) - the BFGs are $130 each in a 205/50-15, vs. $80 or less each for the MXs or Azenis.

That will give you enough to start being competitive with some of the STS guys - and faster than those who are not good for sure!

CivicSiRacer is right - STS is a fun class. It's generally well attended, and you'll probably have hardcore National-level contenders all the way down to beginners like yourself.