2002 Toyota Corolla Replace Just Catalytic Conver or Direc

My car check engine light is on bca of Catalytic converter. When I check with the mechanic. He says that he is going to charge $200 to replace just Catalytic Conveter. Online I could see Direct OE replacement part(which comes with pipes along with attched Catalytic conveter) available for $160. So I am just wondering that Can I buy Direct OE replacement and replace myself.I am assuming that it easy to replace Direct OE replacement myself.

Mechanic said the he is going to cut the pipe and do welding to join the new catalytic conveter. He says that he is going to give gurantee for 2yrs/24,000.

I need you suggestion to decide which option is better? And how easy to replace the Direct OE Replacement?