It’s hard to say without knowing more about the number usage.
But if it is just for searching and identifing a party I will add them as Identifiers of the party as this fields are using when searching parties.
It is strange to have specific sequence for customer. Probably what you are requesting is what we already discussed on this post.
I think you will need to code here. The best is to have a cron job that post customer invoices with a date today. You can use notification_email module to automatically email the invoices when post.
If this is something generic to the whole country I think the best is to extend the german localization modules to include the process to generate the required file format or to talk with the webservice. We’ve done that in the Spanish localization and it works very well as all the features all available for all german customers.
There is a Work in Progress to include a developer tutorial in the oficial documentation, you may be interested in having a look at it as its quite advanced.
Lets see if we can make it ready for the upcoming release.