If, by "iPad", you actually mean everything works entirely in the Apple ecosystem, then I think you're looking for an Apple Airport Extreme, into which you plug in an external USB hard drive. There is also the AirPort Time Capsule, which is essentially the same thing but with a hard drive built in.

If it has to work in other ecosystems (Windows PCs, android phones, etc), then I'm not sure how well the Apple-centric AirPort will work. For instance you mention FTP, WebDAV, and DLNA, and I'm not sure the AirPort Extreme does that. But it will be super simple to set up and it will work perfectly with the iPad.

I personally do this via a Synology NAS connected to an existing network, and the network is accessible via WiFi. But that is a more complex setup, which doesn't fit your "70 year old non-techie" requirement.