As a person who mostly uses very expensive scopes, I second the recommendation for "try something cheap first."
What you're trying to do isn't very hard if you read up and copy what others in hobbyist communities are doing. As other have said, looks like 200-400x total mag is enough. On a microscope you'd be looking for 20x and 40x objectives. There's also a 10x lens in the eyepiece, so you multiply the two together to get total magnification.
Biggest problem cellphones have is them always trying to autofocus, so that $20 camera sounds great, I'd couple it to a simple holder like the one Amazon autosuggests
so you can have a cheap "stage" to adjust focus while your critters are moving.
There are also lots of scope eyepiece adapters on thingiverse that allow you hook your cellphone up to a cheap scope. But you can easily take a cellphone pic through any eyepiece, it just takes a sec to get the focus right. We do that all the time in teaching labs.
Let me think a bit more, my old dept used to do K-12 outreach with a cheapo microscope that was basically a webcam and they actually worked surprisingly well, but I'm forgetting the brand right now. There are a couple guides to hacking a cheap webcam to do this too.
Also check out Manu Prahash's Foldoscope project: https://www.foldscope.com
. 140x and you can easily take pics with a smartphone, and you're supporting a cool project.
If you wanna buy a cheap "real" microscope, I have friends who've had good results with the Amscope brand. But also I'd strongly consider buying used, labs are constantly throwing out old dissecting scopes and inverted light scopes. I've bought some things on Dovebid before, kinda like ebay for industrial/scientific stuff. Trinocular (or a side-port on the scope) lets you permanently mount a camera, usually via C-mount (the adapter has the 10x lens in it). If you can get a used "tissue culture" inverted light microscope that can do phase contrast, that would be nice for transparent organisms. Most of those would accommodate camera that has a c-mount adapter. megapixels doesn't matter much.