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

given your focus on portraits and a plan for more lenses down the road, I don't think it is a bad idea. Rather than getting a kit lens that doesn't do dramatic portriats I'd be a lot happier with the prime.  i bought the OMD with the zoom (14-42) and have never taken it out of the bag.  i've wasted money on the kit lens.  that said, at least the kit lens only adds $100.  You can add the 14mm down the road for very little...

It wouldn't be my choice if I had to live with one lens forever, but you've stated that isn't your plan.  Many people can't imagine living with no wide lens, but if portraits are you thing you could run a portrait studio with just the 45mm