First of all, you might be interested to see our new .NET SDK that also contains new WPF SDK.
What kind of capabilities you are planning to use in your application? If you are only showing offline data (not editing) you can work with Basic license level that is free but if you need more advanced capabilities, you need to unlock Standard capabilities either asking users to log into ArcGIS Online by their credentials (if your users has ArcGIS Online subscription already) or your can embed license key directly to the app. You can read more about licensing from here.
