in case you didn’t already know: email has a STEEP learning curve and it’s because of how it can be captured from outside – not the technology itself.

Where should i buy my domain and are there any things i should be careful about?

Just becareful about renewals. Sometimes a domain will be cheap year one, but crazy expensive year 2+. I personally use Porkbun

does the end of my domain (idk the specific name but .XYZ) matter at all?

Sometimes it can matter. If you host email, some TLDs are classed as higher risk and could be wholesale blocked or classed as higher risk.

Am i willing to self host too much stuff?

That’s a question of how much time do you mind spending configuring, maintaining, and upgrading. I run way too many services. Some of them require more effort than others. Forgejo requires more effort to get working than Immich for me trying to get Forgejo Actions working well.

Where should i buy my domain and are there any things i should be careful about?

As someone else said, check the renewal price, not just the price for initial registration. And obviously try to pick a registrar that you find reasonably trustworthy.

does the end of my domain (idk the specific name but .XYZ) matter at all?

It’s called a top-level domain (TLD). TLDs are handed out by the IANA to various entities. Those entities (eg. a branch of government of a country) can set requirements for registrants and I think they also influence how much domains under that TLD cost. So you might want to pick one with sensible requirements (usually they’re pretty lax but eg. some countries may require citizenship or registering a business with them) that you’re able and willing to meet, is cheap, and that you more or less trust the managing entity of.

I get that i should use a used PC (i have none lying around), but how old?

Depending on your hardware requirements etc. and what you want to achieve by self hosting, I would suggest looking into renting a cheap VPS (edit: embarrassing typo) instead of buying an old computer. It’ll be more expensive in the long term, yes, but you won’t have to worry (as much) about things like getting a static IP address, power usage/outages, noise and possibly heat in the summer, fiddling with router settings, telling everyone your home IP address and sharing bandwidth with your server. A third option would be something like a raspberry pi, if it’s enough for your needs.

Am i willing to self host too much stuff?

The limiting factor, aside from possibly hardware if your services become very popular or are particularly demanding, will likely be your patience in figuring out how to get things working. Email can be a particularly bad one, especially if you need to be able to communicate with mainstream email services. Of course everything also requires occasional maintenance, which again takes time and patience.

Can i use Gentoo, Devuan, or other non-systemd distros as an OS?

Sure, why not? But for a beginner I would recommend picking something with a good amount of documentation and support available to make finding relevant information easier and to (hopefully) be able to rule out the choice of OS as a potential cause when things don’t work.

Welcome to the party, you’re going to have a blast (and lots of chances to bang your head against the wall)!

Here’s my take on your questions:

1. Doesn’t matter at all where you get a domain. Pick any reliable registrar where the price is right.
2. The top-level domain (TLD) is the domain name’s suffix and you can use any that you like. Some cost more than others, so you may want to compare prices.
3. To start with, anything from within the past 10-15 years should be good enough. There are trade-offs, though. Older machines may use more power, for example, but don’t get hung up on details at this point. Get something cheap like an old laptop with an Ethernet port and you’ll be good to go. Once you’ve done it for a while, you’ll understand any downsides and the next piece of hardware you get will be an even better informed choice.
4. Not at all. Self host as much as you want, but I’d recommend starting slowly. Pick one thing and set it up on your server. Tweak it until it’s just how you want it, makes backups, test. Then add the next thing. Immich, forgejo, and a website are nice easy ones to start with. Communications platforms like chat apps and email are much more involved. Maybe save those for slightly later.
5. For sure! You can use any OS you’d like. I’ll mention that a lot of the very basic tutorials will assume you’re using Ubuntu or Debian, both of which use systemd. For Devuan, for example, you could mostly follow those same tutorials, but you’ll have to fill in some of the blanks. I’ll mention that Linux isn’t the only option, either. If you don’t like systemd, you might enjoy something like FreeBSD. It sounds from your experience like you’ll be able to handle this part.

Whatever you do, just remember that it doesn’t need to be perfect immediately. You’re going to mess a bunch of things up and make decisions you’ll wish you hadn’t. You’ll spend a lot of time destroying and recreating things you’ve already done. That’s the joy of self-taught self hosting!