Sure, you can use a VPN if you want to spin up the instance and connect to it without having a domain. You can always open the instance of Nextcloud to the internet later, when you buy a domain.
Get a cheap .XYZ domain if you just want to experiment with spinning up a reverse proxy.
Cheapest would probably be the Raspberry Pi/Orange Pi route.
I gave up trying to make an HTPC (using Kodi, ChimeraOS, etc). It wasn’t worth the hassle and ended up settling for an Apple TV.