I'd like to focus on your depth of field problems and how that relates to ISO. If you're only concerned about one "depth," per se, or distance from the camera, this will not be a problem. The reason for this is that no matter how blurry the background is (or isn't), your subject will always be in focus. However, if you're interested in also picking up subjects closer than that distance, you'll want to make sure that you don't have too low of an f/stop number (too wide of an aperture). If you're using a small sensor, or a wide angle lens, this should not be a problem if you keep the number above f/1.4 or so. However, if you choose to use a telephoto lens to zoom further, you could run into problems with a narrower depth of field.
I'm assuming though, since you're collecting these photos for scientific/historical/data purposes, you'll be less concerned about having the "blurry background" bokeh effect, and more concerned about having everything as sharp as possible. This is where ISO comes in. To achieve a higher depth of field, with more in focus, you'll have to use a smaller aperture (higher f/stop number). That means there's a smaller hole in the lens for light to come through, and you'll need more digital amplification, or ISO. Newer cameras generally do better with ISO than older ones by a good deal, as this technology has been improving dramatically in the last few years. A quick google search with any DSLR name and "ISO performance" will give you information on how that camera performs when it has to amplify the signal. Note that as ISO goes up, noise gets amplified as well as signal, hence the need to check the performance out beforehand.
That said, for your purposes, a used Nikon D3100 with a cheap lens would probably do the job. (I'm not familiar with Canon, so I can't speak to their lineup.)
Finally though, a distance of 90-120 feet is not outrageous by any means. I don't know exactly what you mean by "lowish light," and I don't know how recognizable you need the people to be, but if it's not pitch black, and you don't need to do facial recognition, almost any modern DSLR could get you what you need there. Your next consideration might be price point (and whether you can use the same sensor for video, although that's off topic here).
Considering price point: Look at phone cameras. For example, a used iPhone SE can be had as of writing on eBay for around $80, and seems to do fairly well in other than pitch black. One bonus here is that the sensor size is so small that you don't need to worry about depth of field. For any aperture that the iPhone can do, anything past 6 feet or so will be in focus.
So in short:
Check the resolution. Under about 10 MP you won't be happy with. 12MP is better. For either of these, just make sure you get a 35mm-adjusted field of view of around 24mm or higher. This is still a pretty darn wide lens. 20MP or higher is a bit overkill for your requirements, but if you go that way, you can go with 10mm or so (although these are usually special lenses that you pay extra for, they're so wide).
What depth of field do you need, and therefore what aperture and ISO should you be looking for? If you go with a DSLR, just get your run-of-the-mill cheap lens, and you'll be set. Fairly small aperture (f/4 or higher), and a good field of view for what you're doing (24-55mm should be about right). Don't worry about this for phone cameras.
Look at cheap options. For what you're doing you don't need a $30000 camera. You don't even need a $3000 camera. You may not even need a $300 camera. Depending on whether you need to do facial recognition, and how dark exactly it is, something as cheap as an old iPhone could do the job perfectly. (Bonus: video capabilities).