The most efficient in cost and performance for this scenario is Microsoft SQL Express. It is free and performs very well in a single user scenario. You should install it and run it before you try to connect to the server. Also, it supports data base attaching by the client on demand. You can read more about this feature in this MSDN article. You can compare the different SQL Server editions here.
All the best,
the Telerik team
Q2’11 SP1 of RadControls for WinForms is available for download (see what's new); also available is the Q3'11 Roadmap for Telerik Windows Forms controls.