I just purchased a V70XC. The seller told me that his mechanics told him it needed a new fuel pump. The indicator being that when I turn the ignition switch it doesn't crank the first time but does on second try. My questions is does it need a new pump or a fuel sender unit? The expense difference it huge.

 

You say it doesn't crank on the 1st ignition try. Do you mean it doesn't start? I'm also not sure why you're second-guessing a mechanic - check the fuel pressure if you're not convinced or talk to the mechanic to find what diagnostic was done. A fuel sender issue would not cause starting problems, just fuel gauge reading.