We had a discussion about a legacy 100-135mm lens for EOS-M. There, a few valuable lenses were mentioned; the prices were close to $300 for those glasses. This is close to the native ef-m zoom or a good (second hand/new) EF portrait lens. What's your opinion about these?
-Ef 85 f/1.8 + Canon adapter
-Legacy 135mm f2.8/f2.5 + 3rd Party adapter
It would help to know a bit more about your intended use for the tele on the M. I've used all three configurations and found myself choosing the first (EF-M tele) more often than not for general use since it provides the best balance and feel for me. I prefer to use the M without a tripod in most cases.
I have experimented with a variety of EF lenses in the second mode (35L, 100L, 135L, 8-15L, 24-105L, 70-200L, Tamron 150-600), but find I only mount my 8-15L, 100L and Tamron 150-600 these days (there really aren't EF-M alternatives for these). Probably not a coincidence that the long lenses both have IS since I shoot handheld with them M. I do mount the Tamron on a tripod/gimbal sometimes and the M goes along for the ride.
I mounted FD lenses in the third mode (wide and tele), but that's mostly as a novelty to play with manual focus and see what I can do with my old glass. Wide seems to work reasonably well, tele can be problematic without a tripod. I haven't found a compelling need for this, but there may be some specialized uses with the right lens such as a wide angle manual focus alternative to the Samyang 12mm f/2. The lack of IS on a legacy tele makes framing and focusing with the screen difficult when handheld so you might need to use a tripod, even if your shutter speed is fast enough to eliminate shake during exposure. A future M with some form of view finder might help the situation.