Limiting available downloads per member per subscription period

I'm setting up a membership site with digital downloads. You can subscribe to different levels and each level gives you a maximum of downloads you are able to make each subscription period (a month).
How can I make these restriction in the system?

i.e You can buy level 1 subscription for $10. That gives you access to all content to brows and look but you are only able to download 5 files during the period.

I would like to be able to offer the customer to upgrade to next level (Level 2 $15 Max 10 downlaods) when he reaches his limit. He has the option to wait until next month when his subscription is renewed and he has 5 downloads again.

Hope you can help me with this. I really like your Plugin but this is a vital part of my business.