There are a few things I'd consider:

• How many users are going to be on the MC server? MC is pretty notorious for eating RAM, and since most of my home server adventures often includes multiple VMs, I would look for something with at least 32 gb of ram.
• for plex (I'm guessing similar is going to be the case for Jellyfin) how many users do you expect to support concurrently, and how good are you at downloading in formats that the clients support direct play for? Most remote plex users are going to require transcoding because of bandwidth limits, but if you have direct play for most of your local clients or have a good upload and don't have to transcode 3+ streams at a time, you're probably fine with just about anything from the last 10 years in terms of CPU.
• also re: plex, do you have any idea in terms of storage requirements? Again, if you're just getting started < 10 tb of storage in mind, you can get by with most computers.

Anyway, to give you an idea, I run both of these and quite a few other things besides on a Dell R710 I bought like 4 years ago and never really have any issue.

My suggestion would be grab basically any old computer laying around or hit up eBay for some ~$100-$200 used server (be careful about 1u's or rack mounts in general if noise is a concern, you can get normal tower-case servers as well) and start by running your services on that. That's probably just about what all of us have done at some point. Honestly, your needs are pretty slim unless you're talking about hosting those services for hundreds of people, but if you're just hosting for you and a few friends or immediate family, pretty much any any computer will do.

I wanted to keep things very budget conscious, so I have the r710 paired with a rackable 3016 jbod bay. The r710 and the rackable were both about $200, and then I had to buy an HBA card to connect them, so another $90 there. The r710 has 64 gb of ram and I think dual Xeons plus 8 2.5" slots. The rackable is 16 3.5" slots, so what this means is I basically don't have to decommission drives until they die. I run unRAID on the server, which also means that I can easily get a decent level of protection for drive failure, and I don't have to worry about matching up drives and all that. I put a couple of cheap SSDs in the 710 for cache drives and to run things I wanted to be a little more performant (MC server, though tbh I never really had an issue running it on spinning disks) and this setup has been more or less rock solid for about 5 years now hosting these services for about 10 people.