catted

Just found out the car I bought at the weekend has no CAT and its far too loud. The engine runs ok apart from a slight flat spot. No engine lights and it passed an MOT last week. I want to refit a standard CAT and change the system but will I need new lamba probes etc as well? Are these...

im getting a down pipe made up in a week or so, going to get it priced up tomorow. only thing is what size do i go for? 3"? i have a scorpion cat back exhuast. think thats 2.5" :confused: also will decat make my car stupidly loud, or just a bit louder than it is?