What does the Coast Guard plate read on the transom? Is the boat still in good condition? What HP is it rated for?
By your description so far I would think more on the lines of a 15 horsepower engine. Will you use it on salt water? That might make a difference in choice of brand of motor.
BTW Johnson and Evinrude are both OMC products so it's which one you like the looks of better. On the older ones, the Evinrudes were quieter because of the hoods on them.