Sharepoint Programming

We have a small company that finances inventory for independent car dealers. We would like to develop a password protected website, using the SharePoint platform, that would allow customers to log in, see inventory, payoff inventory and pay invoices. All of these things are currently managed on excel spreadsheets. The SharePoint website would need to duplicate the existing inventory listing and functionality (2 examples attached), existing daily calculations (fees current period, curtailments due, outstanding principal, days in inventory, credit used). It would also need to automate the invoicing process of which there are two: First - There is a recurring weekly invoice cycle that occurs on Monday morning (00:01) for vehicles that have "aged" on the floor-plan for a 30 day period (30, 60, 90 etc). Second there is an episodic payoff invoice that occurs when the dealer selects a car in inventory to completely pay off. The math is basic, there are a total of 17 customers, so this is not a complicated piece of work, but it is important to us! So send me your best share-point proposal and we can discuss. This will be a fixed price project, which if done well will certainly lead to maintenance and upgrade opportunities.