Hi, my daily activity is to work with excel, I have developed some complete and complex software, like POS for restaurants and retails, made completely on excel .(you can see...here) [login to view URL]
I fit the requirements you needs to make this project (as you need only win XP)
I can develop it using a MySQL database and Excel, showing a look and feel as you painted in the mock-up. .As a plus, I could give you a login user and user profiles controls, which is important because you will be working with money and invoices implied. It could be installed as a client/server software, so, many people could be working at the same time with the software.
Please, do not lost your time reading bids which promise you to finish it less than a week, if you accept those, you will be running a big risk about the quality.
I hope to know about you soon.
Regards.