Something FAST. Nothing's more distracting than a unresponsive touch screen. If you can, go to a best buy or catr studio store and bring a USB cable for your phone or a thumb drive crammed full of songs. See if the stereo can handle it. My 4 yr old unit lags pretty bad with a lot of music on it.
Other than that, try to find something with a volume knob because using volume buttons without steering wheel controls sucks.