It sounds like you’re trying to justify buying a car you can’t afford.

Find a dumpy little local muffler shop and they’ll probably weld on a cheapo aftermarket cat for less than $500.

It might also not even be the cat - an exhaust leak or a bad o2 sensor can cause the same error codes. Might just be a couple hundred bucks for a new sensor or some gaskets/welding repairs. My old car was throwing a catalytic converter code and a $3 gasket from autozone fixed it.