does a buyer get a notification when a seller updates their model?

Yes

Among other things, a model that I purchased here last year just got updated, and I received the notice that it had been updated. Is good as I just downloaded the revised model.

Like @LemonadeCG said! From time to time I change something minor on the model, descriptions or something similar and on some old models probably more, like update. It would be good to have a separate update button.

Perhaps the notification is sent only when the product itself is updated. Not when the product description or images are updated.

Yep makes pretty sense Forester. Changing one word from italic to bold... and spamming a buyer with "revised version" would be a big no go.
I guess it applies only if new files are uploaded (fbx, obj etc.).

I guess it applies only if new files are uploaded