Look, take it from someone who knows.
1/ redacted - ok the vendor is a dealer
2/ you paid beer money for a champagne product.
3/ Your own presence here will brand you in Court as an expert in MB
Your only recourse is the one open to all people, trade sales or private, which is that a vehicle must be "roadworthy", which does not mean it must pass an MOT, it means it must not be a danger to other road users, and you've been complaining about peripheral stuff.
You forked out £10k on a dog, get over it, clean the car up, MOT it, write a GOOD advert (but do not lie) and stick it back on ebay asap.
This (last car I sold) is a GOOD advert, if it had been like the advert for your merc it would have gotten maybe 25% of the selling price it did.
YOU made a judgement call, YOU are not a novice, YOU got burnt, this happens, now ask yourself if you are going to continue to spend hundreds of pounds on peripherals on a car that has an almost expired MOT, or are you going to get shot and get your money back and buy a car you will be happy with?