Becoming a Nurse While in The Army?

So I'm 18 and a half years old, just graduated high school a month, and leaving for Army Basic Training in 2 weeks. I'm going to become a Medic in the Army. My long term goals for my life is to become a Nurse in Army Nurse Corps by the time I'm around 27...So far my plan is to enlist for 4 years, then do reserves for 4 years, and while I'm in the Reserves to become an RN. I hope to become a OB/GYN Nurse. What I want to know, from a Civilian or Military, does this goal seem moderately realistic? How much of my courses can I do online or during active duty? Just a general question, roughly how much does it cost to become a RN? I'm not so much worried about the money because I qualified for a Army College Fund that will more than likely completely pay for my education.