I once owned a '79 280E, the '80s are somewhat different so my advice
may NOT apply here.
I recall a rather large (2 cm) white plastic or nylon set screw - head
slotted for a large flat blade screw driver. This is located on left
side and seen from the front of the engine with the air cleaner removed.
This screw is the idle speed air control and is what you need to adjust.
This engine should idle about 700 RPM, in Neutral.
Can't help you a bit on the LPG question - Rob Smith may be able to.