You need to know what you are getting into. Obviously Merc will be superior and therefore more expensive to keep. You can sell a kidney to get a merc fixed or earn more to keep one. Else, get a jap banger.
Toffee you are experienced to know all this but I wouldn't buy anything I cannot afford to keep on cash. Yes, you got cheated but most brits need to accept this now, each time they buy a car from a trade and they part exchange, they get ripped off. And I have witnessed countless times now. And I feel sorry for people who say "when i saw the car I fell in love with it or I know this is the one".
Each time I buy a car I account for worst case, if its a lemon how to make lemonade out of it. Having said that, do not let them thiefs get away. Good job on getting your money back. Most people don't have the knowledge or guts to do what you pulled off.