I think it would be easy to count the number of downlaods and check it against some limit set in the config screen.

OK. Thanks. If I get to add this to the code, I'll post it here.

Anonymous download will be tricky as the module completely depends on the user being logged in.

I kinda guessed that..
I do think there is a strong case for it, as many sites don't want to mess with users and passwords, and just want to sell a file and maybe another thing (e.g. a book PDF and a printed book). Having the link being sent in email to the users is a great convenience and a click-saver..