I need clarification on what type of Catalytic Converter I can put on my Audi.

I need to be able to pass DEQ in Oregon and want to know if it is smart or acceptable to install a cheaper catalytic converter to get through DEQ. I am confused because I see several possible catalytic converter options on the internet but they seem cheap compared to what repair shops want to put in my car. Does anyone have a suggestion as to where I can find a relatively decent cat to buy? Also is is true that the exhaust manifold has to be replaced during this process? Also, what is the expected cost of this repair. I'm guessing 1,000-1,500.