Started with one RPi 3, ended up with 5 in a case that needed ventilation and a switch. It looks cute, but... The only one working to my pleasing is PiHole. Nextcloud is slow as hell (you are bound to external HDDs over USB and that sucks). 3 use normal HDMI ports, 2 mini HDMI. When shit hits the fan and SSH doesn't work for some reason, I have to plug in a monitor and keyboard.

Oh, and one SD card went poof due to not noticing it had no free space left and still writing logs on it for 2 weeks. SD cards are unreliable in general.

I regret not using VMs on a more beefy mini PC that I could have upgraded to my pleasing, benefit from SATA, and would have been easier to maintain.

So I would recommand RPis if you actually need and use the IO ports. Otherwise, you will soon learn they get overburdened. For general self hosting, myself would have gone the ProxMox route (which has a free tier and that's what I have experience with).

I don't think it has enough ram for even 1 of those applications (except pihole), not to mention it'll be hard setting up an external drive on it.

best get some kind of 2nd hand thin client, it'll be much much better.

For PiHole it will be OK, but for Nextcloud or Jellyfin is too slow. It is better to buy used MiniPC/Terminal - it will be more powerful

No as that hardware would be way under powered for almost any of those tasks on their own, let alone for all of that on one machine. Your best bet would be to look at the hardware recommendations for each service you are interested in and go from there.

From a pure performance standpoint a pi zero is good enough to start and learn the basics. Nextcloud and pihole should be working just fine. Regarding Jellyfin depends on what your usecase is.

In any case, I would suggest to start and see for yourself. The you can as you go.

E: To boil it down: Pihole - enough, Nextcloud - very simple file handling, jellyfin - streaming music yes, videos not much

Pi hole will be absolutely fine. Nextcloud is unusuably slow on my Raspi 3b so it's definitely not going to work on a zero. Jellyfin is definitely a no also unfortunately.