I thought type/technology of data storage was unimportant.

It makes a big difference to development decisions. Example - if you were doing an app for dotNET and SQL Server, you might decide to use SQL Server stored procedures to do much of the work. Then your app would not work at all with Oracle, MySQL or local tables. So you'd need to be very sure that all your customers will want to use SQL Server. If you wanted to offer lots of backends without having to write special code for each, you might look at the unfashionable but extremely easy Remote View technology offered by VFP. Or using dotNET, you might set out to write special middleware to write the data access part.

If I were to offer generic advice, I'd recommend that you take a good look at creating a browser app using ASP.NET. It's comparitively quick to learn and looks good.