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.