Personally, I think there is a limit when it comes to buying used stuff, and buying a used computer is my personal limit when I have to spend my own money (don't take any offense, please!) With that being said, after my experience with AppleCare/Genius bar, I would not buy any computer without warranty. However, if that is on Ebay, I would get the Squaretrade warranty with that. Squaretrade will refund you the price you paid for your computer or try to fix it.
For a new battery, that is $130. I'd rather buy a refurbished white Macbook that will comes with warranty, a new battery and it will look brand new. You won't have to worry about a thing. Sure, it comes with slightly lower specs, but you might not notice it depending on what you are doing with your computer and for me, personally, its worth it.