The difference between Application Builder & Solution Developer Licenses

I run the IT department of a non-profit, and we currently use MS NAV 5.0 with the additional Serenic modules for non-profits(NFP). We would like to develop an e-commerce website that will connect directly to our NAV db, where we currently store customer information, product information, etc. etc.

Our main objective is to create the e-commerce in .NET. Users will fill out forms on the website, and after going through the business logic, if the data is approved/accepted it would hit our NAV db. The data would either populate fields that are already in NAV, or we could create new fields in the current tables, or even create new tables to connect. Plus we would also like to create a new tab in a record. For example, in the customer record, we would like to have a new tab called RESTRICTIONS that restricted customers from ordering certain items.

My question is what development license would we need to complete the above? I think the Application Builder is enough, but I am not sure. I understand the Solution Developer License lets you do everything but is also around $28K. For a non-profit that is a lot. Any suggestions, or thoughts would be greatly appreciated.