P2000 & P0420 code

Hello All,

I am trying to flog my Hawkeye 06 Impreza Sports Wagon 1.5R

Currently experiencing constant CEL's with it. Everything is ok around town, but 5miles on the motorway and BAM: P0420

I ignored it whilst I was using it as my daily and checked again a month later, CEL has added code P2000 to the list.

A while back the front O2 died, probably put 2000miles hard miles on the thing on holiday, just hope this hasn't killed it.

Car drives fine now that the front O2 has been replaced but I'm trying to sell the car and no one's interested in a car with CEL on.

Gonna clean the EGR valve, pray that works.

Has anyone got any other suggestions? If it's the front cat, that means replacing the whole manifold -_-