I'm a qualified mechanic and trust me i wish i did plumbing haha! Mechanics isn't such a good job to be honest, I would stick with plumbing if i was you.. Better Money, Better working conditions ect ect the list goes on, but then again if its what you want to do no one has the right to tell you not to, So do what you feel is best! But get plumbing under your belt first would be a waste to chuck it in now