How do I sell (premium) usergroup memership that expires automatically

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On my site, I would like to have a paid section, that will only be accessible by member who belong to the premium usergroup.

I want to sell this membership in packages of 3/6/12 months or something like that.
I don't need integrated payment processing, this is unfeasible because users on my site will use a non-mainstream payment processor that I would not bother to integrate as it would involve to much work.
The workflow I have in mind is:
1. user pms payment receipt+payment code (supplied by payment site)
2. I assign a specific amount of premium time to him, in the form of usergroup membership (important)
3. In due time, the premium membership expires and the user is automatically removed from the premium usergroup.

Are there any modules that do this, or help with this? If not, would it be hard to develop?

JR