Based on what I have seen on the show and read on the boards I don't think Travis is going to propose to the bachelorette he selects. He may give her a ring that represents the hope and promise of a possible life together.
He seems very methodical and doesn't want to commit to something that may not work out in the long run. After just a few dates how can he be 100% sure this relationship will last?
He is going to choose Sarah S. She is the girl he can begin a relationship with based on friendship and mutual respect. There is a romantic connection but he wants to move ahead slowly and cautiously.
By selecting Sarah S. he doesn't have to uproot anyone from the life they had before the show, as she is just a few blocks away.
If the relationship doesn't work out he may move to a different state to practice medicine. He wishes Sarah S the best and moves on.
I am not saying their relationship won't work out, just that he has his life planned out and she has to fit into that plan.