That is a lovely portrait, Helen. It
is a little soft though. I wonder how much of that softness comes from your Summarit?
Rangefinders have ALWAYS been about fast glass. Fast glass reduces the "need" for high ISOs. Get the Nokton f/1.1, but keep your Summarit. The Nokton is pretty large (comparatively speaking) to tote around daily if you don't need it. I had both the f/1.5 and f/1.1 versions at the same time and made the dire mistake of selling the f/1.5 thinking I would just use the f/1.1. I don't.
My recommendation would be to keep the Summarit and try to find a Nokton 35mm f/1.2 v1. I also have one of those, and it is my go-to lens on the M8. It's a little larger than the f/1.5 lenses, but not as large as the f/1.1 and on the M8 gives the 50mm FOV that you're used to. My carry 'round setup with the M8 is the CV Ultron 28 f/2, the 35mm Nokton f/1.2, and the 75mm Heliar f/1.8. It's a light-weight, but effective triad that gives the traditional 35-50-90 FOV combo.
I don't usually shoot above ISO 800 with any camera. I shot this night scene hand-held with the Nokton f/1.5 at 1/15 at ISO 160 with the M9P last summer. I didn't record the aperture, but it was probably around f/2 or so. I find that low-light shooting takes a little more work perhaps, but is absolutely do-able.
L1006273 by
chief1120, on Flickr
The shutter sound isn't that of a III body (or even the M4-P,) but it just sounds like a Copal vertical focal plane shutter sounds. Shoot with it a while and give yourself the opportunity to get used to it. You'll soon find that it's really not that objectionable to you, and your subjects in front of the camera won't notice it at all. If it really annoys you, you can set it to "discrete." Some folks say that helps... I don't find it much different. Having shot cameras like the Besseler Topcon Super D, the RB-67, and Hasselblads for years, a 35mm Copal metal focal plane IS pretty quiet to me. <grin>