Canon 5D pictures blue
White balance solved - good.
But perhaps the "problem" the 5D might still have could be the mirror coming loose. I don't know of any way to check if it has been repaired. This issue is essentially a recall though and Canon will repair it. So I would check with the seller to see if they know if that problem has been fixed. It sounds that you were able to make a relatively good deal so it might be worth while to consider sending to Canon service even if it turns out you might have to pay a checking and cleaning charge at worse. At best they will fix the mirror at no charge.