As for could or can, "could" would most often be used in the case that you assume the person is capable of doing it and you want to know whether or not they'd be willing. "Can" would imply that you want to know about their ability to do so. Generally you wouldn't ask this; if fact, it sounds quite strange.
To be still more polite you might write:
I wonder whether you might be willing to help me.
Of course, it is more direct to simply to say: "Will you please help me?" or "Would you mind helping me?"