Supporter for Users

Hi,

I would like to ask you for a plugin - which will work with users not with blogs.
I have a networks of 20 blogs on my site and only users can register.
Users automatically have Contributor role and I would like to give them possibility to buy Author role. So free acount is with contributor rights and paid with author role.

It coul be nise if I can setup for paid account that in their post wont be ads (like adsense) but in free account will by automatically.

I would like to sell the paid membership for a time period - years.

Can you help me with this? Can I setup this with some plugin of yours?

Thanks,

Bigdrobek