Best ultra low power microcontrollers are the XLP series by Microchip, and the MSP430 series by Texas instruments. The Arduino microcontroller is not famous for being ultra low power, but if you sleep (or use the lowest power modes) much of the time, you can do surprisingly well. A small FPGA can also do pretty well, depending on what you're trying to do, because they tend to run at low voltage, and everything runs in parallel, so you don't necessarily have to use really high clock frequencies.
You should make a power budget, and try to figure out if you can make it work. I can tell you right now, the motors probably are going to be the big energy drain. Also, if you're going solar, it's best to do it in bright uncloudy direct sunshine, otherwise your power generation is not going to be very good. My gut reaction is that you're going to have to go with some kind of battery option, otherwise you're not going to be happy.
