Keep in mind that you're asking for suggestions, which are inherantly opinions. I'm sure you've heard all of the old lines about asking for opinions, most of which don't qualify for the "PG" sort of posts we encourage here on DBForums!
I've got a slightly unorthodox suggestion, but one that seems to fit your needs wonderfully.
MS-Access can handle a dozen people nicely, but it does not scale to Enterprise grade applications very well. It can usually be bludgeoned into working by well disciplined experts who are willing to spend many, many man hours working to overcome some of the scaling issues that Access has, but this isn't anything that I'd suggest.
MS-Access does make a great prototyping tool for Enterprise applications. It allows you to quickly build, analyze, and experiement with your ideas to see what works and what doesn't. It even allows
SMALL scale testing (a few users at a time).
Best of all, MS-Access will happily use a MS-SQL (Microsoft SQL Server) database. This database engine will scale to Enterprise tasks. Several very large databases on the Internet today use MS-SQL.
What I would suggest is that you get your application designed, prototyped, and functionally tested using MS-Access. Once you have things working as you'd like, I'd create (or hire someone to create) a web site that delivers your MS-Access application as a web site. Odds are very good that this web site would be created using one of the tools in MS-VS (Microsoft Visual Studio) such as C#, or Visual Basic. If you (or your developer) have strong preferences, you could certainly use other tools to build the final web site.
Before you start to build your application, you might want to read up on
SaaS. There are a number of things that you can design into an application that will make it easier or impossible to deliver without significant changes. Understanding those things up front can save you hours or months of work before you can start to sell your product.
-PatP