Unique Subscriber ID's?

I’m trying to do something that seems fairly simple but not having much luck finding a plugin or non custom-programming solution. It’s a simple membership/password protected content issue, but with one extra requirement that’s stumped me:
1. When someone becomes a member of the (yet to be built) site, I would like to be able to assign them a member number. It needs to be a certain format that I have control over, as this number will then be used to track purchases from the vendors we represent. The vendors demand the member numbers follow their existing numbering system.
2. The number will auto-increment for each subsequent registered user.
3. The number will also be used to access certain site content.
4. The member database would need a special field to store the number.
5. Otherwise, the membership plugin would only need:
a. confirmation upon joining
b. bi-monthly emails to members
Does anyone know of an existing product that can do this?
Thanks in advance!