Help – How to add a custom customer number

My Client wants to be able to set a separate Customer Number for their clients, different from the auto-generated id. The requirements are:

• Ability to filter/search Customers by that number
• Display that number on the Invoice PDF
• Ability to edit that number in the admin interface

Since I’m a newcomer to magento development I was hoping I could get some pointers as to where to start solving this. I don’t think there already is a module that should do that? (At least I couldn’t find any)

I’ve watched this tutorial on module development and have found it very helpful, but am still somewhat struggling to figure out what the best solution for this problem would be.

Aby help is greatly appreciated.