On this 'fast lens vs slow lens' debate, lens performance is not always a factor in selecting one over the other. All my Zuikos are the slower lenses because I value compact size and lighter weight and, as a bonus, all have the same 49mm filter size. Whatever we decide all choices invlove compromises and trade offs. Sometimes the littlest thing makes a big difference. On my OM-1 bodies I leave off the hot shoe, it is amazing what a preceptual difference that makes in the feel of the camera even though it is a very small part.
About the 40mm f2. I've had 3 pass through my hands, all purchased for much less than the then current adverage price. The last one I bought several years ago for $200 and sold for $600. That lens is just too valueable to be a user. All my Zuikos would be rated BGN or UG by KEH but work fine.
In regard to OM-1 vs M body size. I've had OM-1 for 40 years and when I finally got an M4-2 in 2010 I was surprised at how small the OM was sitting back to back with the Leica, only the mirror box makes it thicker and the prism peak is barely 1/2 inch higher.