Creating invoices from a client list

I am new to spreadsheets and Excel 2008 (mac) and would appreciate advice on the best method to accomplish something. I have a large list of clients that I would like to be able to create invoices from individually. At the moment I have mail merged these contacts but each time an invoice is created for every contact and I am unsure as to how to specify a particular company.

Ideally, since there will be other people also using this client list to create invoices, I would like to create a drop down list or similar, where I could choose the company name and the address, contact details etc. are automatically completed in the invoice.

(Just to give you a little more information to help me with, the spreadsheet is a very simple one - Company Name, Address 1, Address 2, Town, County, Postcode, Tel. No., Fax)