You could always get one that uses MP3 DVDs. But I believe the JVC DV6200 does both USB and MP3-DVD. If you check the manual I'm sure it'd say what it'd run off the USB port. As far as power is concerned, the USB spec is rather specific at 5V 0.5A (though most are fused for 1A)
Edit:
Got the model # right, but they don't have the manual online yet. Though a couple clips from the product page:"Take your Thumb Drive/USB Drive and load it with up to 1GB of your favorite songs. " <-- looks like there might be a 1GB limit (of course, I have an RCA MP3 player that blindly ignores the 512MB SD card limit when I put a 1GB SD card in it...) "Dowload DIVX Videos? Go ahead and watch them on this unit." <-- that's sweet, even though I'm never gonna put a display in my car.