I think that Yamaha has the best reputation/re-sale. The make the top end of the Merc. four strokes.
Honda should also be considered.
Some thing to consider is if the valves are in a pocket in the cylander head. his protects them if the timing belt skips. I think Hondas and Yamahas are.
A poster on another site had MAJOR problems when he hit a rock.
One cast can change your day...maybe your life.