It's probably "Add python to environment variables." They must have changed the wording on that at some point.
Not on Windows right now so I can't confirm, but you probably forgot to pick "Add Python to PATH" or whatever the option is in the installer. Try running the Python installer again, maybe it'll let you add it without needing to uninstall & reinstall
Edit: If you're on Nvidia, there seems to be a simpler install method now: https://github.com/AUTOMATIC1111/stable-diffusion-webui/wiki/Install-and-Run-on-NVidia-GPUs (Method 1)
how much space was the download(s)?
On my end, it's sitting at ~64GB (with btrfs compression shenanigans), though 60 of those are from all the models I have installed. The download would probably be ~2GB, even less if you disable downloading the "default" models with --no-download-sd-model and instead pick models off of Civit or wherever manually.
Edit: Should have mentioned. Most full models are between 2-4 GBs each. Some can be 5+ but they tend to be "full" versions intended for merging & such. LoRAs are generally smaller. Depending on how much they're pruned they'll be anywhere between 10-100 MBs each.
how confusing is the software to use?
There's definitely a learning curve, yes. But there's plenty of resources (and more importantly, examples) out there.
what kind of limitations does the software have? can i do multiple people? monsters? futa? etc.
As long as you have the correct models set up it can generate basically anything. At least with anime models, monsters and futa are a given. Your main issue will probably be multiple people, although there are solutions to that. (See the multidiffusion upscaler GitHub repo on the main post)
By Web UI it means that the graphical part of it -- where you write your prompt and hit generate -- is running inside your browser and not as a separate window or command line. Everything is kept on your own computer unless you explicitly tell it to open up remote access.
This would be impossible without directly modifying Lemmy itself.