1. Im running an i7 10700 with an RTX3080 10gb, 32gb of RAM and I get 70fps with 1 person in scene, 30fps with 2. That's with custom settings and a really small scene with basic light setup.
2. I'd suggest allocating 500gb for VaM. Currently at 300gb for the whole file size. Cache @ 100gb. You can clear it if you want, or delete looks and scenes that you don't use anymore.
3. Also had that dilemma when i started, it's not rly that confusing-the hub provides. (unless you don't get it there, and your sources are from creators in afdiian or arca.live). I keep track using folders inside AddonPackages so I don't get confused. Some provides all the dependencies, some don't. Up to you if you deem a var file with missing dependencies as usable or not.
You can ask for dependencies in Asset Requests if you're missing any.
https://f95zone.to/forums/asset-requests.96/
4. Virus-wise? So far I don't get any sketchy files from the hub. I use the hub in the browser and inside VaM itself. Can't say the same for the ones on patreon, afdiian and arca.