Olympus OMD EM-5 body + the 45mm as my only lens (is it possible?)

Look at the photos you've previously taken. If the majority of them were taken at wide angle to normal focal lengths, then it would be stupid to only use a short telephoto lens, would it not?

If you're on a budget, the 14-42mm kit lens only costs an extra $50 to $100 with the camera, so it's extremely affordable and it's silly not to pick one up.