I suspect kwikfit, while simple/convenient, would be a relatively expensive source for a cheapnese POS that'll rust out within a couple of years. If you want cheap and don't care about quality/fit/longevity then you'll find some properly (as in the raw materials would cost you more) cheap examples on the bay
If you want something that's OE quality, decent longevity and actually fits properly (without lots of swearing and sometimes blood) then you want Eberspacher, Ernst or Bosal. Typically waaaaaaaaay cheaper to buy from Germany for example a back box my 320te...
MB part # is 1244910300 which is an Eberspacher 23.386.80 an Ernst 374 15 6 or a Bosal 278-235
Stick any of those into the search engine of your choice and you'll get a bunch of hits including
End Silencer (EBERSPÄCHER , 23.386.80): MERCEDES-BENZ Parts - Bestpartstore.co.uk which was the first hit for the eberspacher part # and is a German company despite the name. That site will give you MB and manfuacturer part # for your particular model
FWIW a few years back MB were actually pretty competitive for exhaust bits and i've got a vague recollection the middle box i bought from them was made by Ernst. Personally Eberspacher would be my choice, fitted one of their back boxes a couple of years back and it's better finished than the MB example; aluminised coating which is still mint while the MB middle box was painted flat black and the paint was already falling off when the parts muppet handed it to me
Or get a 304 stainless system made up if the cars a keeper
By 'whole system' do you mean whole system or just the middle and back boxes? If yours has a cat then decent quality front section is 'kin spendy for obvious reasons and cheaper will just mean a cheapnese POS cat that'll probably pack up/collapse within a couple of years
