An old laptop is a great place to start. If you want to use proxmox, I would first check the bios settings to see if virtualization can be enabled for your processor. From what I’ve read some dell laptops don’t have this feature in the bios. But even if you can’t use proxmox, the laptop would be great for learning a linux system. And if you want to dabble with docker it would be great for that as well. You can set up many services with docker, including nextcloud and plex. Containers would be better suited for an older laptop like this anyway.
8Gb of ram would be more than enough for those 2 services. If you find you find you are outgrowing that ram, only then would I think about upgrading. You can get a lot done with 8Gb if it’s only for home usage. I think your biggest bottle neck would be your CPU if you are transcoding video using plex. But if you are only streaming to 1 1080p device at a time, this should work without a problem. Plex uses next to nothing for ram.