Straight to your questions:
1. In order to avoid sending a request to the server before user types the minimum number of characters you can set minLength property. In order to show hints you can bind the keypress event to the input field.
2 and 3.In order to achieve such behavior you can use the DropDownList dataBound event. To execute your custom logic only when the data source is filtered you can check whether there are any filters set using the filter method of the data source and get the filters.
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