I have used in that order when hiking
E420 + kit lens
EP2 + kit lens,
EP2 +12mm+20mm+45mm
EM5 ( either with 14-45 +25mm when multi days hiking, or i add 12mm+45mm+75mm if day hikes )
At full size i have seen an improvement from E420 to EP2 to EM5 and from kit zoom to some primes,
But even the E420 quality is accepted from Shutter / IStock / Dreamstime etc... most of the time.
With the EM5 when its not accepted its never from the camera limits
i tried a few shots with LX3 and had some accepted on the 3 above, but it way harder.