Semi newbie with semi ambitious goals, point me in the right direction
iAmTheTot@kbin.social to 
I am interested in tech and would say I have more knowledge about networking than the average person, but there's still a lot that's way above my head. I've played around with Pis a little and have set up things like home assistant and foundry VTT before.
I got my hands on an old machine from work and want to upgrade it with some of my spare hardware, and ideally have it set up as my single home server. I would like to host plex, a few foundry servers, home assistant, and use it as a NAS. Basically combine everything I have right now into one more powerful machine.
Thing is... It feels really overwhelming to try and start. I'm not sure really sure how to go about the whole endeavor. What OS do I use? Do I use VMs, docker containers, is there pro or cons one way or the other? How can I make sure they're all accessible from the web securely?
Is there anything you guys can tell me to get me started? Some resources to walk me along setting a multipurpose home server?
I was on the same boat few months ago. Went for Debian OS and then docker containers for almost all services. Portainer is awesome web UI to manage all containers so thats what I install first. Starting up new services is about copy paste docker-compose with slight modifications for your needs. My server is accessible through wireguard connection and that should be one of the most secure ways and probably the easiest to setup afaik (downside of wireguard is that you have to setup each device to allow connection - AKA scanning qr code)
Just saying what I did, most people here are more experienced and Im still learning.
GL and HF
I feel you. I was where you are 2 years ago and it was really hard to start off.
I'd suggest starting installing Debian as an OS. Then, go for Docker.
If you feel lost, you can take a look at some videos from DBTech on YouTube. Look at his playlists to sort what you need.
Remember to document everything you do, because you'll forget what you did to make things work your way.
Have fun !
You could use a complete solution like TrueNAS or https://www.openmediavault.org/ but IMHO if learning is equally a goal you could also start with a standard Debian or Fedora Server Edition. Personally I use the latter and it is very nice to use with the Cockpit web-console and the good integration with Podman for running Docker containers.
I can't point to any decent resources, but in your shoes I'd probably download a Debian based distro that's similar to what you used on your pi's (Ubuntu server or Debian itself), learn how to use docker (see the other post where a user is asking about containerisation today for community responses) and set up a reverse proxy like Caddy to safely host your content on your lan and once you've got it working on there then think about internet access and whether you want to go down the VPS/Cloudfront route for public access to your goodies.
Given how Plex is trying to diversify away from self hosted content, give Jellyfin a spin - it's surprisingly good and supported by anything with a browser, iOS, android, firestick, kodi or whatever!
Oh and by the way, it’s easy to feel overwhelmed starting out in this. My advice is to set yourself small, achievable goals to help break it up. Start by doing your hardware upgrade, then work out what your host OS will be and install it. Then work out how you want to use it, what you want to containerise, what you want to run in VMs.
be prepared to knock it all over and start again. Everyone rebuilds their environment, usually after learning how they could do it better. And most of all, have fun!
Definitely used to this already just messing around with my few Pis lol.