You may not need the focus on the lens. It would be good to immediately (almost) react to user input, without waiting for other key presses or mouse clicks.
I have implemented a search text control. It listens for the TextChanged event and when it fires it starts a timer to wait a (customizable) time in milliseconds. When the timer expires, it fires an event to say the caller: "hey, search the text". If the user keeps writing, the timer resets, so there's a nice delay between writing and searching.
It behaves like the vista/7 search box. When you type, it shows the cross/close icon. I can sell you this nice control if you want