I built my first thing from scratch. I say "thing" because it was neither a NAS, nor a server, or a hypervisor. It had storage, standalone services, containers and VMs.
If you are unclear on what you need, DIY is probably the best approach.
I've invested a lot in my DIY machine. It's a node in a proxmox cluster. The other two nodes are mini PCs. If I was starting from scratch now, I would probably go with a prebuilt NAS and a bunch of mini office PCs for running VMs and other things.
DIY gives a lot of flexibility but you are managing every aspect of it. Borking the storage, borks your whole setup. It's a lot of fun but you need to know what you are getting yourself into.
Getting only a NAS will be insufficient once you start experimenting. Which means you will need to get another machine for hosting services/VMs. It can get expensive quickly.
If you go the DIY way, start witha hypervisor and virtualize everything else. That way you will have a more stable setup.
It's worth the hassle. If you are obsessed with power consumption, consider the fact that Pis are very power inefficient. They draw less but also produce way less than other CPUs. Performance per Watt is shit. They were conceived as thinkerware. They are not reliable the way everyone here wants them to be.
Just to put your puny power concerns into perspective. My homelab draws around 130w on average. One big box with lots of HDDs, 2 Lenovo Tinies, networking.
My laptop draws more power in 24h than my homelab. If you have a desktop, the discrepancy is even bigger. Nearly anything in your household blows your setup out of the water. I live in a country with relatively high power costs and I can assure you, your concerns are nothing more than a thought experiment.
I would get the N100, get 2x2TB SSD as well as some smaller SSDs for VM and LXC storage. Create a hypervisor with the appliances you need - NAS, Docker, heck if the board has a free PCIe slot you can put a 4x1GbE NIC and spin up a networking VM too. This way you can streamline the maintenance - updates, backups, etc. And the best part is that this single box will be waaay above anything you have now.