Seems to me you have the real benefit of having over a year before you need to make final decisions, so my suggestion would be to spend that time learning all you can about your loan options and the different ways to invest (property, stocks, your own business, etc.). The JL Collins series tends to be a favorite here for explaining investments in the market, and of course there's MMM himself.
In the interim, make sure your GF maxes out any tax-sheltered accounts available. Keep your expenses and lifestyle low -- if you can live off of her income now, can you continue to do so once you graduate and throw all of yours into paying back the loans and investing? If you can avoid lifestyle creep, you will basically have the game won.
The biggest revelation to me about the MMM lifestyle is that it reverses the retirement math. If you plan a 40-year working career, the most important thing you can do is get into the market as early as possible, because the vast majority of your retirement kitty will come from compounding (this is the staple of the common "if you start at 25" vs. "if you start at 35 or 45" examples -- each extra decade of compounding [more than] doubles your stash). OTOH, if you are looking to retire in, say, a decade, compounding has very little to do with it, and your RE date will be driven almost entirely by how low you can keep your expenses and how much you can sock away every year. MMM has a helpful chart of how long it will take to be able to retire at specific savings %.
Tl;dr: Take your time, read, learn, and figure out what combination of lifestyle + savings will be right for you. That is 90% of the battle right there.