I installed a Pioneer DEH-P5000UB and have been really happy with it. The install is caveman simple, but I'd keep an eye on the anyhow. Mine has a rear USB/iPod connector and it controls the iPod from the front panel, or you can use the iPod for control like you do now.

After installing this, the subwoofers really came to life. But many different headunits may be just fine.