I just bought a used Lenovo ThinkCentre M710Q Mini Tiny Desktop PC Computer i5 6400T 1TB SSD Win 10 Pro from Ebay for $289 AUD and plugged in some oldish external SSDs and HDDs and now have 10TB of storage. I’m really pleased with it, it took about half an hour to install Proxmox and I’ve now got 5 VMs up and running.

I wouldn’t recommend a WD My Cloud Home - it’s not a NAS as such, it’s a bit limited; I’d go for a Synology. or One Drive as you suggest - a 1TB plan is quite reasonable with regards to cost.

I use uptime kuma for monitoring - really easy to set up and very versatile