Learning German, then going to school there?

My Oma lives in Mainz, Germany, and when I graduate from high school in December I'm going to live with her for four months to improve my German, then I'm coming back to the states. I'm either going to go to school here in the states, or I am going to spend a year in Germany living in Mainz and taking classes at the Goethe Institute preparing me to pass the DaF so that I can attend school at one of the universities. My goal is to be able to obtain a work visa after school and stay in Germany if it works out. My question is, do you think it's possible that I can reach enough fluency in German to attend a university there? I've taken 4.5 years of high school German, my dad is a fluent German speaker and is currently only speaking to me auf Deutsch. I would love to be in Germany for school; this is only a dilemma because if I choose to spend that year in Germany to learn the language, if I haven't reached enough fluency and decide I can't attend a university there I might not be able to start school in the states until I'm 21. I don't know what that would mean for ACT scores and etc, but basically I'm choosing one or the other. Yes I've thought about studying abroad. The schools that I'm applying to in the states for my major do not offer good study abroad programs, the longest period of time I can participate is up to a year. So. It's one or the other for me, I don't want to ruin my education to switch countries, but this is an opportunity that is more available to me than other people and I'm not sure about due to the circumstances. Any information from personal experience, or any suggestions? Also, my Oma is getting old and I don't have a lot of family in Germany still, so because of that and if I'm paying American college tuition, I won't be able to visit often anymore. I have also researched extensively on the DAAD website and they do not offer an English co-taught program in anything that I want to study. Help!!