@Bladt, if you're considering a 135mm lens, don't get a Canon P.
Here's an RF accuracy chart from
@splitimageview:
The Canon P will struggle with a 90mm at f/2, and can't be relied upon to focus a 135/3.5 or 135/4 wide open; same with a Contax IIa due to the shorter RF baselength. And, if you're shooting music and theatre, you're going to be using them wide open.
Based purely on the use of long lenses, I'd be looking at a Leica III, Leica M3, Contax II, Kiev 4, or any of the early Canons with the multi-position viewfinder set to 1.5x magnification. Yes, with anything other than the M3 you'd be using an external viewfinder, but that's really not the problem people often make it out to be.
Arguably the best option would be an M3 with the goggled 135mm f/2.8 Elmarit (extra low light capability + focusing accuracy), but that's probably out of your budget.
As for shutter noise: that's going to depend a LOT on the condition of the camera. My Leica IIIf sounds a lot louder than my IIIg; my Contax IIa sounds a lot louder than my Contax II. This is just down to wear-and-tear and bad services they've had in the past (the IIIf was noticeably and worryingly louder after Aperture in London got their hands on it). That said, the Contax and Contax-based shutters are a lot louder at slow speeds than the Leica-derived shutters, so I'd write those off as an option anyway.
For my money: a Leica III, a Canon 135/3.5, and a good quality dedicated 135mm finder. You could do a lot with that and the outlay wouldn't cost you much compared to some of the other options.