Before you throw parts at it measure the oil pressure to ensure the gauge or sender isn't reading incorrectly. You can tap into the port on the oil filter housing for the sender pretty easy. With the one piece oil pan, replacing that pump will be a nightmare with the engine in the chassis.

The idle problem is likely vacuum leaks.

When buying a used 560 it's always a good idea to change the oil and filter and throw in a set of the RECOMMENDED plugs inspecting the old ones for signs of oil fouling. Use good oil with a quality filter as the M117 valvetrain wont tolerate anything less.