I've wanted to make a headless audio player for my car, and this power issue has been the bottleneck for me. Additionally, spinning disks and moving cars are a bad combination.
Just last week I bought a 500gb SSD and usb adapter for about $75. My Pi 2 powers it without issue. The only problem I see is that the SSD can get fairly warm, so keeping it in my closed center console probably isn't a good idea.
