None at all tbh, at least if you use the PC alone and don’t share a lot of stuff with Windows devices. If you do, then maybe scan .exe or other files (e-mail attatchments, etc.) with ClamAV or similar to prevent spreading stuff.

You usually don’t need AV software because you install stuff differently than on Windows. You don’t hunt .exe-files from random internet sites, thats irresponsible even for Windows.

You install your apps directly from your software center (a frontend for Flatpaks and repo software), where they usually are pretty safe.

Also, sandboxing is a thing. The prefered way for most people (and often default) is via Flatpak, where apps are restricted on what they can access and do. You can lock them down even further if you want.

There are more ways of sandboxing, but those are not so relevant here right now.

Also:

• If you run a script, check it first. I have zero clue in regards of coding, but even I can usually guess what each line is supposed to do.
• Don’t add 3rd party repos if you can, use containers instead
• Go for the easiest route, guides for “Linux” aren’t noob friendly. In your case, search for “Mint” instead, most stuff is pretty easy there.

100% AMD, for sure. AMD won’t make much problems and works ootb.

Nvidia on the other hand… if you already have a Nvidia GPU, then the proprietary drivers work pretty well, but even those won’t work flawlessly and still cause problems for many people.
And the FOSS drivers are still in the early stages and won’t cut it. So why spend lots of money for a piece of hardware that won’t give you the performance you paid for?

Also, Nvidia clearly doesn’t care about PCs or its’ users, so why support such a shitty company with your money?