Best option is
Flex ----> WebORB ---> .NET Classes
WebORB is easy to manage.
.Net also easy for startups
Backend is usually a client decision so you really need to know how to deal with most of the options, if you have clients that don't want licensing costs a backend with php/mysql is usually the way to go.
Ok, I'm more familiar with. NET, but in case I'll use that tool? Can I use Visual C # Express only without having to purchase Visual Studio to integrate with flex? Using WebORB in the case.