When you are in need for a script this complicated/detailed, it's always best to do it yourself, or get someone to do it. The downside is that it would turn out quite expensive if you don't know how to make it. As far as I know, there isn't any free (or even paid) script that does exactly what you need.
And just another thing - you said payments would be done offline, but why? That would most certainly limit the number of buyers (which you probably don't want). To overcome this problem, taken that you are making a custom script, you can implement automatised PayPal and eGold payment systems. I have done this before, and it saves a lot of effort
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