I would have thought that if there were no faults shown for 200km that you probably have a poor connection somewhere, a faulty cat would surely have produced an error almost straight away. I think most of these codes are persistent, that is they stay on till cleared, so a bad connection would cause the light to come on and stay on. A good idea to fit a new AFM, in my experience they do not last long and are often responsible for uneven idling and poor throttle response, just ensure you use genuine Alfa or Bosch only.
I bought a second hand cat for my JTS which cost me £120 including delivery plus a further £25 for gaskets, I was lucky and it was just fine, indeed the recent MOT test showed the lowest emissions the car has ever produced. If you could get one fairly cheaply it is worth a try, if it doesn't work well try another one. I suggest that firstly though try a new pre-cat & post-cat lambda sensor costing around £200, much easier to fit.