"Just a few more questions: is this a fairly straight-forward DIY job for a person of moderate ability or should it be done professionally?"
I tend to do these $1000 jobs myself and if I'm strapped for time will let the shop do the $65 brake fluid change though I did do my last brake fluid change myself. Two months ago I replaced a head gasket and I spent all day Thursday and a Friday night after work till midnight finishing it up. I do these so infrequently that it takes me longer that it would someone familar with the process.
If you haven't done it before consider getting together with your local MBCA section and make a driveway-mechanic session out of the task. About a year ago we did exactly this with a Virginia Section member's 300E or E320.
In either event make sure the timing chain is properly bleed and tensioned when you finish.