Ooooh, that is very interesting. I have not come across the models with inbuilt light meters, nor constant speed motors with that little box on the side. That might well be the transistor? A person likely to be able to tell you will be Sean Charlesworth, who was head repair at NYU looking after their sizeable 16S collection.
I'm afraid I cannot advise you on the power supply with a view to the light meter. But I think you're absolutely right to err on the side of caution; light meters are sensitive bits of equipment.
Both the variable speed motors and the governor controlled constant speed motors were kind of reliable in their frame rate. But the most reliability requires a crystal sync motor - although that transistor model was apparently pretty good - good enough for the B-GS to run sync sound, in any case.
As regards power supplies, the camera itself can take a bit of range: 6V for 18fps, 8V for standard 24 or 25 fps, and up to 12V for 48 fps. Do not use 12V with the 400ft magazine unless you have the magazine torque motor altered to take this. Also, it's not advisable to run the camera at 12V for long amounts of time and without film; it can really damage your motors.
I'm sure you'll have come across the options people use most frequently. A common DIY solution is a rechargeable RC battery, which tends to come in 8.4 V. They're fairly inexpensive. See this topic for more info.
That's the set-up I'm using at the moment. I have a couple of issues with it, though. One is that these kinds of batteries don't hold charge for very long, and likely only last a few years. They sure hold enough charge to run the camera for a decent amount of time, but if you're needing to replace them every few years, isn't it better to find a more durable solution? Also, my RC battery fluctuates, from 7.8 to 9.4 V. Might be age, but it's not great. I could technically install a voltage regulator to at least step down a higher voltage to an exact 8V, however, that doesn't allow to boost up, plus most voltage regulators require a minimum difference of 1.5 V between input and output to be effective.
Michael Leake has a homebuilt battery pack, which he explains in this forum post here. It looks like he uses a buck convertor voltage regulator, judging by the photos.
I think if you have V-Mount batteries lying around for other purposes anyway, that's a really solid way into powering the camera, but it's not a solution I've come across a lot in this forum, for whatever reason. Alvin Morris demonstrates a version of it in this video here. He's using a Tilta plate. There are a few other manufacturers who offer 8 or 8.4V outputs on their V-plates, but the Tilta is definitely the better rated one.
Theoretically, you could get a basic, cheap V-mount plate that doesn't output 8V, and install a voltage regulator between battery and camera to get the exact output you want, but I don't what you'd need to take into account with a view to the camera's impedance. I imagine the boards at Stack Exchange might provide some wisdom...?
Let me know how you get on. Has yours been converted to take XLR input, or are you one of the lucky few to still have the original ARRI plug on the back?