The lowest power lowest noise machine you can get would be a powerbook. 
If you can come up with a pismo, lombard or newer machine, that is 
about as good as you can get. The batteries are rated for about 1400 
mAh, with some power saving enabled and the screen off I suspect that 
it would draw even less than that from the wall. Maybe you can pick up 
a pismo off of ebay with a broken screen and hook up an external 
monitor for when it's needed or serial console to it.