For the first part, if this will be for repeat customers, I'd think about
using a master customer sheet where you can add all of your customer
information, then use a combination of Data Validation for customer reference
and VLOOKUP to return all of their relevant information.

If it's one time orders and you just want to build a database of orders
you'll need to go the other direction and use VBA to transfer the invoice
data to a master sheet.  

But you need VBA anyway to increment an invoice #, which you can do like so:

Range("A1").Value = Range("A1").Value +1

HTH,

Smitty