Well, I'll come out and fight my corner now!
I am the aforementioned technician with the Merc. Turned out to be a problem with the auto box, (hopefully) solved by a gearbox oil and filter change.
I upgraded from a 1ltr Fiesta to this car. I can afford all the payments needed for it for it so I thought "why not" as I'm still young!!! I was looking at getting something like Focus for 6 or 7k but then this car came up at an attractive price with all the history and in good condition. Knowing the seller helped too. I thought it would be cheaper to run that for a few years than all the payments associated with getting a newer car so I went for that.
As for Merc problems, mine is right on the borderline between the reliable years and not as reliable years. Admittedly, it isn't exactly hewn from granite but it's still reliable compared to some later models. I don't mind having the odd problem as I enjoy fixing stuff like that (I remember changing the universal joint on my fiesta, open carburettor in my face reaching down the back of the engine, fun fun!).
As for the rust, my is totally clean and free from it Even the front subframe is in good condition as I found out laid under the thing on Saturday!