Update: The fuse on the OVP relay checked out OK and I ran/test-drove the 300E for about 45 minutes, for two days in a row, without problem. Previously, it hadn't died long enough to be able to check it out more thoroughly. And NO ONE here in Yuppieland has METRIC fuel line fittings with a 14 mm male thread to make an adapter for my fuel pressure guage. Will need to search my local salvage yards and cut one out of a junker.
Or maybe I'll let her leave it to the local MB dealer!
Happy Motoring, Mark