Re the macro question, I find the 100mm focal length is great for getting some working distance between yourself and the subject. I find this better than the working distances provided by shorter focal lengths like 50mm. Less likely to spook insects or you or the camera shadowing the subject. Helps also with flowers that are are just a bit distance in gardens where you cannot just stomp around in the beds.
You will observe there are a lot of macro lenses made in the 90-105mm range. Some people use even longer focal lengths but these are serious tools costing serious dollars. However extension tubes with tele lenses with get you 'close up' if not 'macro' images easily.
Auto-focus is arguably not that important with macro as focus depth is so shallow that some manual finetuning of the focus target is quite possibly necessary anyway. If you have a M lens, then you are already familiar with green button metering. I think I am encouraging you towards a M 100/4 macro like I have - good sharp lens that won't break the bank, moderately common, and nicely built. Note though it goes to 1:2 ratio, not 1:1. An extenstion tube will give you that extra magnification though. 1:2 is challenging enough to start with though! An F2.8 lens is nice as the viewfinder is brighter, but the actual shot is more likely to be taken at F8-11+ to get adequate DOF, and the larger aperture pushes the price up.
Be cautious of the macro claims of zoom lenses. The term macro is used very loosely by zoom manufacturers - close focussing maybe but not true macro.
I need to do more macro when time permits - I'm discovering that good lighting is perhaps one of the key things needed to get good macros, and may need to invest in some lighting to really improve. Surprisingly though, even the camera's on-board flash can be useful - I just use trial and error to get the right aperture/ISO for the distance I am shooting - F11/16 with ISO 100/200 is a good starting point.
Back to your M100/2.8 - out of my M acquisitions, just one of those lens seems 'not right'. Looks clean, but the images are soft and lacking punch versus other M lenses. Annoyingly, it was one of the more expensive ones. I may try to seek out a repairer for it over the next few months - if so, I'll let you know the result. Rest of the lenses have been very good - good to excellent nick and image quality consistent with the reviews on this site.
I had a quick play in poor light with my M 135/3.5 a week or so ago. It can be frustrating with CA at times (partic in the OOF bokeh) but also creates some lovely shots as well when all the stars align. Common and cheap, I think it is a bit of a bargain. I'll post an image or two to this thread tonight:
https://www.pentaxforums.com/forums/pentax-slr-lens-discussion/230356-no-love-135mm.html 
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