I've been through this exact ordeal before. I went and bought a catalyst from some random exhausts' company and it was the biggest mistake i'd ever made, was noisy as hell and it didnt pass the MOT.
But if the catalyst on your car has gone, i've seen more than one person on here say that you can remedy it by opening up the catalyst (much like a tin of bins), emptying it out, and welding it shut. This eliminates the noise and aparently it can pass an MOT (cant speak from personal experience though, as i haven't done this).
But what i decided to do was get a sports exhaust system, which i purchased from MIJ Performance, £500 pounds from the manifold all the way to the back, including the nice looking stainless steel tips. This is slightly noisier than usual tho

. But i owned the car for another 31miles after buying this system and i have to say, it ran like a dream, and passed the next 2 MOT's before i part-exchanged it for a w202 c230K.
Definately beats having to pay £2000, or in my case, even just £870 (when i was quoted at the time) for the catalyst.