Depending on your state, emissions shouldnt be a problem assuming your motor is in normal shape (IE not overly rich or burning a ton of oil, etc).

I would go Carsound or Catco personally. Although I have no experience with OBX cats specifically, I would prefer a company that is more "proven" when it comes to a cat. Do it once, do it right. Cheaper cats tend to fail prematurely by clogging or having the material inside come apart and rattle.