IMO, you want ram more than you want processing power. 16 gig ought to be enough. Most of the time your containers will sit dormant and just consume memory. However since you want to run Jellyfin, get a recent CPU which can do hardware decoding of popular codecs. There’s charts online that show what generation can handle what codecs. Ideally you don’t want that done by software. You should still be able to find something cheap.
In terms of placement. It depends a lot on noise IMO. If you’re running something small without magnetic storage, you’re probably fine to stick it anywhere. If you have several data-centre grade hard drives, you will probably want to keep it somewhere where you wont hear it all day.
In terms of upgrading, I’m not sure if its as much of a concern as you might think. I run probably about 30 docker containers off a NUC clone and a seperate NAS, and that has worked pretty well for the last few years. I can always add more drives to the NAS, but otherwise its fine. Also, many of my services scale to zero with sablier+traefik, and I schedule filesharing for low bandwidth times. This makes things pretty manageable.
I agree. About 10 years ago I had a some unstable dependencies hit in the middle of a major crunch/product release at work. When it was vital I was productive, I was instead trouble shooting my laptop. I moved to mac the next day and was surprised how far the OS had come, and that I could run zsh, nvim etc. Not to mention since apple silicon its rare I need to take a charger with me anywhere.
I still have a linux thinkpad for personal use, and all my personal servers are linux. My heart is linux, but a lot will have to change to take me away from a macbook.