How can I remotely access a Mac for elderly assistance using Apple Remote Desktop?

Screen sharing across the Internet, either with the service built into macOS or via Apple Remote Desktop, is possible but tricky to set up and get working properly since it usually involves special setup in the routers on each end. While not usually free, a third-party solution that uses a server to moderate the connection, such as LogMeIn's GoToMyPC is often a better, or at least easier to maintain, solution.

Another possibility, if you think your mother could manage it, would be screen sharing via FaceTime:

Share your screen in FaceTime on Mac - Apple Support

That would require her to initiate the call and do all the work on her end; you can't take control of her Mac that way.

Others here may be able to suggest other remote control apps/services to consider.

Regards.

"Is Apple Remote Desktop a good choice for this?"

Not really and primarily for the reasons varjak paw mentions.

"Any other apps I should be looking at?"

I would not bother and use what macOS already provides. I currently support a friend who lives a similar distance away using the Messages App. Provided you both have an AppleID you can request to take control of her screen using it. At her end she either allows you to take control or blocks your request.

Below screen grab shows you the options available: