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.