I think you already know, AIOs are the go-to, just make sure you can connect in. I’ve done this with Synology, works fine, I used sftp to sync things. If you want cheaper you can look into a standard linux host and mergerfs/snapraid, but it’s going to be a much higher learning curve, and a much higher risk of failure. If you’re just getting up and started don’t overthink it. It’s good to plan for tomorrow, but think about how much data everyone has, and how much you’ll use today, and then double that. That’ll be a good baseline.

If you’re US based, a trick, buy the WD Elements drives from Best Buy. They go on sale regularly pretty much whenever there is a holiday sale and “shuck” them (plenty of videos on Youtube for how to do this). You’ll save probably double the cost on drives.