Here’s what I know:
I wouldn’t use existing wiring, would use 20 amp lines with correspondingly heavy gauge wire, would have an electrical contractor that gets the inspector to permit/ approve it (and if possible, somebody who has had experience wiring stuff for studios or halls).
Unless you have the ability to get a completely separate service for your system, you are, as far as I know, going to be sharing ground with the household system, which has the potential for noise. There are tricks about which leg, etc. but the idea of a big-assed isolation transformer is a good one.
Do more outlets and lines than you think you need- way cheaper in the long run than having to break walls and have a new mess on your hands (with all your delicate gear, records, etc. whatever already in place).
I have an Equi=Tech big box sitting in storage- I thought I’d be relocated by now; there are some specialty panels made by audiophile companies, but I suspect the real hands-on folks may say Square D brand or equivalent is fine (that’s what’s currently being used in my NY house, 60 amp subpanel, plus a 240 volt line for one isolation/step down plus an additional isolation transformer for my tone arm air compressor).
I’d look at electric as part of an overall plan for the room- are you doing separate air conditioning system- Mike L followed certain practices used for studios to keep air rush quiet; also, I’d think about whatever current state of the art is in filtration to minimize dust but that’s a peeve of mine;
lay-out for signal cable routing;
lighting and how that relates to electrical- lights can be noisy;
I actually got rid of my AC conditioning boxes once I put in the dedicated lines, I thought the system sounded better without conditioning;
I’m sure there are other things I can’t thing of now, but happy to talk, I’ve been sketching out my new "room" for several years, in the process of waiting to relocate. My plan is still a separate structure and build-out from whatever house we buy or build and its electrical system, i.e. separate service to the "studio" which can also be a zoning/code issue;
Oh, structural- springy floors suck though you can deal w/ that;
noise suppression- I never thought about sound proofing, but did it for my compressor room, which is adjacent to the listening room, used mass loaded vinyl. That stuff is crazy heavy and there’s some good stuff about how to best use it if you are open to the studs.
I didn't do built in treatment in my listening room, but aftermarket bass traps, diffusion and some absorption. Mike L. probably has one of the more over the top rooms in the States and I know he's refined what he originally had done after living with it for some years, so he'd be good to talk to and is a nice fellow.
And a comfy chair. Like the Spanish Inquisition. :)