From my experience as a sysadmin I see two possible path you could pursue:
- Buying some Nas system from the "usual suspects" like Qnap or Synology, and go with their ecosystem. Regarding virtualisation, you probably would want to use SSDs inside for good I/O. You should use a 4-slot chassis or more slots, so that you can grow, and maybe use for data storage cheaper HDDs that for the I/O-sensitive part like hosting VMs.
- You build your own NAS from spare parts. Nowadays it is easy to get old computers (hint: some refurbishers buy old corporate gear and resell it for cheap with some warranty) and put some linux distro on it, or use some ready-made system like TrueNAS. While you go this road, you should clearly define which amount of data you will have to handle, and which availability it should have.
Regarding energy consumption: For PC systems, that are small, I liked the mini PC types like the Prodesk or Elitedesk Mini from HP- they have basically to form factor of a NUC and are powered by an external notebook power supply. You can get easily 16 GB main memory in them, and swap out the internal SSD to some bigger SATA model.
If the change rate of your data is not high, backup to external USB HDD should be fine.
Alternatively, you check the costs of new hardware versus the energy costs- for getting first experiences about building some NAS it ca be overall cheaper to re-use old PC parts instead buying new stuff, if the old parts are still reliable. So you could go for an old PC, put 2 mirrored SSDs in for the main system, and 2 HDDs for data storage. Some mini-ATX chassis should be sufficient to do this, and even with an old quad-core CPU from the Haswell times there should be enough power to drive that.