Willie, Could you use 2 REALTORS? Did you sign anything? As long as you are not under contract with your girlfriend's agent (or any other agent for that matter), you can choose whoever you want!
But you should also choose wisely.
Rent to own, lease to own, and variations of these are generally more risky and not as well defined in Minnesota law as a Contract for Deed. It sounds like a Contract for Deed would fit your needs as well, but there are certain responsibilities you need to make sure you are aware of before you choose one.
There may be some investors who will let you have input into the choice of property, but they will always want to have the final say and that is why there is risk involved.
When you choose an agent, make sure you ask lots of questions. You need an agent who is not only proficient in these types of transactions, you also need to have someone you can trust.
However, personally, I have never work both sides of the same transaction. In other words, I would not represent both the seller (investor) and the buyer (you) in the same transaction. If it came to that, I would ask my broker to represent one side. That way there is not even a hint of impropriety and each party can then be represented to the fullest.
And that is really what it boils down to. Can your agent really represent you and negotiate for you to the fullest if he is also representing and negotiating for the other side? Make sure any REALTOR you consider explains agent to you completely. It is of utmost importance in your situation.
I hope that helps!