I think one of the issues you might be running into is that a lot of linux guides use terminal commands instead of GUI tools, because it's a lot easier to make a good guide with just terminal commands - the steps are short, you don't need large images and an image editing program for every step, and they're less ambiguous than a guide that goes "open this, click here, open that menu, then click there ...". This isn't actually that complicated once you get the hang of it (at least not for the type of thing you want to do), but it's definitely intimidating for a new user.

Games and documents should be fairly easy, chances are good that you can connect any drive you already have and it will immediately work. Software is a little more complicated, I'd honestly recommend not bothering with that until you're more familiar with Linux. The issue here is that almost all software you'll use on Linux is distributed via package managers (what's called "app stores" on commercial OSs) and wants to be installed to certain standard locations - you can move those locations to external drives, but you'll definitely need terminal commands to achieve that, if only because I guarantee that's there's no guide that won't use the terminal.

On the plus side, IME software (excluding games obviously) takes up less space on Linux than on Windows. 128GB is probably enough for all your software.