I would not look at his guide. If you've watched any of Louis' videos, you already know this guy is a ranting machine. He can go on and on for hours about things. I watched about 15 minutes of his rambling and realized he had gotten basically nowhere. It's also one of the more complex ways of doing things. Use ZimaOS to get started with the easy button.
Stick with whatever router you have, for starters. You can upgrade later. You don't necessarily need that at all.
For the actual server I highly recommend this guy. N100 is very common due to being very inexpensive and efficient. You'll have to add RAM and an SSDs but you probably want to choose exactly how large that is anyway. It has 4xNVMe and 2xSATA, if you decide you want to expand later.