Office 365 will give you some testing of the functionality of end users, but if you are going to develop sharepoint, you really need a localized environment. Sandboxed solutions will take you far, but only part of what you need to truly develop sharepoint.
I suggest starting with the basics of sharepoint in a localized environment. This can be installed on a Windows 7 computer with SQL Server Express and does not require any licensing other than the Windows 7 host computer and can work with 4 GB of memory.
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