1) Plenty of power, after all you have 150 hp now
2) Does the boat self bail with the current motor? Moving weight forward can offset the extra weight of the 4-stroke. Batteries in console, shift fuel tank forward.
3) Transom height will dictate the length of the motor shaft, is the current motor 20"or 25"?
4) It's my understanding that the lightest 150 4S on the market is the new Mercury. If you go with the new Mercury you will save on the rigging.