1. Depends on your luck, could drive forever or at least till next MOT, or a big chuck could turn sideways and block exhaust any moment.
2.Here is the guideline prices for diferenf scrap cats,as you can see prices vary a lot from nothing to over £200
http://scrapmetalforum.co.uk/phpbb3/...php?f=15&t=475
3.If they are TYPE approved I don't know why unless you are 'treehugger' and want to do every last bit to save the environment, but seeing you drive S8 yuo probably don't care about that much. If not TYPE approved they will be MOT failure unless you have a friendly MOT tester.
4.If you replace them like for like with second hand unit for example you can do only one. If going for aftermarket then better do both at the same time.