idk what resolution you use for streaming but my raspberry pi 4B runs plex at 1080p just fine as long as it isnt using x265/AV1 (but on jellyfin you might be able to use the Pi's GPU for transcoding).
I use nextcloud too but it's a tiny bit slower than I'd like, but that's likely a wifi issue i think.
Literally any PC on Amazon for $200 CAD, then add your own SSD. I'd say 8GB of RAM but that's just for cache, youll rarely go over 4 in general use.
That, or a raspberry pi 4B/5 which runs you about $150 once you get a case, power supply, powered USB dock for sticking SSDs into (just for safety since technically the pi's USB ports cant handle certain SSDs power reqs.) and then stick SSDs into that.
Use dietpi (dietpi.com) for setting up your services and it'll run nice and smooth for anything not H265, which might be annoying but Plex and possibly jellyfin let you transcode stuff in the background which is nice.
Shouldnt do so that bad. my raspberry pi 4b can do jellyfin and nextcloud without pushing 15W at full load.
x86 is inefficient, especially older models, but youll likely only push anything over 10W when actually streaming something that requires transcoding. Most of the time your home server is gonna sit idle or doing some tiny cron job that won't really blast the CPU at all.