This is quite a common dilemma, and there is a reasonable argument on both sides, but the most important opinion is your own. You say your Mercedes is lovely, has only 78,000 miles and you enjoy driving it, so you need to ask yourself if you really want to change it.

A known, well-maintained car that has been in your care for some time is arguably worth more than the potential reliability of something else. It is understandable that your mechanic would prefer the Ford, but this might well be because he has worked on hundreds of them because of their popularity and parts are easy to get hold of. Your Mercedes might well be more expensive to maintain, but if it is running well and has been serviced there is no reason why it cannot continue to do so for many years.