I'm returning to aMember after a long time away (my work hasn't needed membership systems) and I'll admit to not re-familiarising myself as much as I should, but I'm busy, lazy and tired. So what's here will be a bit half-assed. I'm wondering about using it for a single product download, without 'proper' membership. Is it possible for buyer to not need to create a login, but to simply be sent a link after payment, which enables a limit on download attempts and time?
Possible but will require customizations. Actually account will be created still but user will not be aware about this. Also this will depends on payment system that you will use.
Based on what you describe, a customization of using their email as the login and auto generating a password seems a good option.