Well the only thing about that is counting on the receptionist to take care of the backup. I think it needs to be fully automated. I figured leaving her with the offline is fine since all she has to do is put in the disk and take it home (she currently does that).
But the online backup I figured a USB drive would likely have the same fate of a computer that is damaged/stolen and she may not remember to do the backup regularly.
A NAS that is in another room at least won't be stolen and would be on a different circuit. Any downsides to NAS? The current lineup on Newegg doesn't get stellar reviews but then again this doesn't need to be fast, it just needs to work. Always.
A USB drive doesn't have to be difficult to manage - you can get portable USB drives in capacities up to 1TB that are bus-powered and are plenty small enough to fit in a purse. You'd want to have at least two backup drives that alternate so that at least one copy is always offsite.
The backups can still be automated, and if the computer is left on then they can run overnight. You just need something popping up on the receptionist's screen to have her swap the backup drive once a day.
For dental records privacy is likely an issue, so you'd probably want to use encryption to try to prevent disclosure of the data should one of the backup drives become lost or get stolen.