For Price/Quality, I would go with something like a thinkpad. I have several that I use for work and home and have not had an issue. As far as battery life the two things that are going to use the most juice are the LCD and the hard drive. So to extended battery life you want to get a laptop with a slower hard drive, but in turn you lose some performance with the hard drive already being the slowest component of your computer.
The ThinkPad SL400is on sale right now for less than $500.
Thinkpads are going to be more for getting work done on the move, rather than a mobile entertainment center.