A five hp is plenty for an 18 foot boat. If the motor starts and runs fairly well I see no need to replace it. Most of my friends who replaced their motors with new 4 strokes still have the same problems they had with the old motors which is mainly fuel. When they go back to the dealers under warranty it seams like they all clean the carburetor. So, make sure you use lots of fuel additive for the ethanol and when you think the boat will be sitting for any length of time run all the fuel out of the engine by disconnecting the fuel line from the engine while will still running.
And yes, I would remove the motor from the stern before trailering or until you feel the motor mount and transom can handle short towing trips.