Do not do anything against your wishes or you will repent all your life. Your father is not going to marry the girl, you are. So you have to decide. Since you both are going to the same University, it makes sense to stay together, to cut costs and have someone from your own country to lean back on.
What you can do, is to first meet the girl before you leave for Germany and talk to her. Keep marriage in mind, when you are conversing and finding out about her personality and nature. Find out from her what she thinks of the arrangements of staying together. Staying on the campus but seperately is a better option.
It is really surprising that your parents want you both to live together without marriage. Make no mistake, living together for such a long time will lead to sex, unless you both are so strong willed and can control your passions. Why don't you talk to your parents and ask them to arrange a meeting and if things are fine, you can both get engaged before you leave for Germany. This way you will ensure there is less gossip from friends and relatives that you both are staying together. If you are not able to meet (if she is staying in another State), then you can exchange photos and can talk on the phone. Get connected together on a Sosial Site, where you can view each other and chat.
Your worries about her bring a year older are unfounded. One year difference is not such an issue. But again, you will need to find out her views on this.
Do you permamently intend settling in Germany? Actually you are too young to think of marriage just now. Do your Masters and then decide about marriage. Make it known to your parents and especially the girl that you will take a call on marriage only in 2 years time. All the Best.