This has happened to my '84 190E, 2.3 twice, once last winter and once this spring (not between consecutive oil changes). Oil pressure would be nil @ idle and no more than 2 bar @ speed. Each time it would happen I would change the oil and the problem would go away. The used oil looked like used oil, and it has not happened at all during the summer.
As to the oil sending unit I believe it is on the drivers side in the front, sort of below the power steering pump, I think. I'll check and repost if this is wrong