For simple backups any drive could be used. I'd go with something like WD blue or similar drive. If they have data that is critically important to them they should also consider some type of off-site backup system or cloud based backup.
Ditto. The Western Digital Blue drive, of your preferred size.
Back in the day, five years back or earlier, I had more spend-thrifty habits leaning toward some standard of "quality" or "reliability." And we weren't as concerned about household power consumption. So I ALWAYS purchased WD "Black" drives.
The Blues aren't "Black," but I have several of them allocated or deployed in various uses. Never had one fail yet. Never had any problem with them.
They may not be the top performer, but that, too, becomes superfluous with certain strategies and software for caching the HDD to either SSD or RAM.