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There can be difficulties focusing manually in low light but Rangefinders (with viewfinders un-affected by lens brightness) are better than SLRs for that. Less shutter slap too, so generally great for hand held, low light.
If anything I find bright, directional light can sometimes wash out the focusing patch.
Metering in low light is no different to any other level. I do use a handheld meter for ambient readings to get a ballpark. Phone apps can do the same.
I'm just imaging low light wide open being difficult without a meter. I'm probably just overthinking it.