I want to go into the USMC and be a mechanic for BMW, what should I do?

I am 17, currently a senior in high school, and also taking an auto cl at North Metro. I am also in the DEP for the USMC, I am scheduled to got to basic June 1st, 08(active duty). I don't know if I should do 20 yrs in the corps and retire, or get a job working for BMW after tech school. I know I would make a lot more money working for BMW cause the car industry is so short on workers(my teacher said he made 80grand his 3rd year as a certified mechanic), but at the same time I think I would like the lifestyle of the USMC a bit more. Think I could retire at the age of 38 from the corps, and then began a career working for BMW or for whatever other company I want to work for? Will the USMC pay for auto tech school? If so how long would it take me to complete school while on active duty? If I were to get a mos for aviation mechanics in the corps could I just retire and work for bmw with out any school