It depends on the research and the work you are wanting to do. The old Testament, was written in Hebrew and later translated to Greek. The New Testament was written in greek because that was the language of the people. If you are planning on doing some scholarly research, I would speak to the advisers in your school because they know the availability of these courses and the requirements for the type of work you wish to pursue.
I haven’t taken any language courses but being at seminary I see guys taking it. Greek is a lot easier than Hebrew and if you want to understand the NT better take greek. The only advantage of taking Hebrew is that you know the OT much better.
Immerse yourself. The more you do, the easier it gets. One of the major problems with learning dead languages is that if you stop pushing yourself, you’ll just forget it. Not many people you can casually chat to in Latin or Koine, in the way that you could carry on practicing a bit of Spanish or German without actually studying it anymore!
Some Koine is hard, some is easy. Once you’ve mastered the basics of grammar and vocabulary, some texts will be relatively easy to read, e.g. John’s Gospel. Others are just a bit more complicated, and will take longer, like Luke.