5sp/150k-Eng revs on accel like slipping trans-power loss uphill. on 2004 Toyota Corolla

Car vibrates in 1st gear when accelerating too quickly. Check engine light on. Dealer says catalytic converter needs replacing $1800 (incl labor). Do I have two problems or one? Failed emissions test.

Asked by for the 2004 Toyota Corolla
It sounds like the clutch disc is worn requiring the clutch components to be replaced. Is the Check Engine Light due to code P0420 (low cat efficiency?)
Yes, the check engine light is on. Is getting an after market direct fit catalytic converter a cheaper option for me? Kinda strapped for cash right now...