If on a tight budget a second hand cat is your best option then your next best step would be the high flow cats. I've got the Berk hfc's and have had no problems passing MOTs the the last 2 years. Though I will say that some members have had problems though and you might find that even though the Berks have the O2 extensions to eliminate the engine management light coming on there's a posiblity it still may come on. I find that mine does, though only if I leave the car idling. Easily reset with a code reader though,or switched off with an up-Rev. Another thing to add is you might get rasp with them with your exhaust,good if you like rasp. I don't but I don't get any with the invidia Gemini I have fitted. I'd do some research before you jump in, but don't let me put you off ! Haha as they bring the car to life