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

If I could only have 1 lens (under $500) on the OM-D, it would be the 12-50. Sure, it's not the sharpest, but it takes great pictures, and is ready to go for many more uses than any prime.

Unless all you shoot is portraits, or never need a wide-angle, keeping only one prime is going to be too restrictive.

Personally, if I were to only own one prime, I would own a wider prime because it would be a little more versatile, if not the best tool for portraits.

As a new Om-D and 12-50 owner who almost passed on the 12-50 because of all the negatives brought up, I am very glad I didn't. The wide end is great, the rest of the range is still quite usable unless you are blowing things up very big. It also does video very well, handles the elements, and can do decent macro.

I would recommend getting the kit, because it is a steal when you buy it with the camera.

Alternatively, buy the cheaper kit with the other zoom lens, AND buy the 45.

The 45 is an awesome lens that is on my list as well, but I couldn't imagine only having that as an option. Your photographic style might be fine with that however, just my 2 pennies.

Sam