Iceboy,
I have an '89 225 which is the same engine that you're looking at, and although it smokes like a bastard and drinks fuel like no tomorrow, it runs well and is reliable. The previous owner put a rebuilt lower unit on it which happens to be counter rotating even though the engine itself was right-hand/clockwise from the factory. In talking with my mechanic there is no down side to this set-up. Just remember to buy a left-hand prop if you need a new one (I didn't
).
Regarding Vezo's comment about "unscrewing", I have no expert opinion, but it seems a bit far fetched to me. A boat is built in a manner that does not favor the prop's rotation. My Seabird was used for 1 season before I bought it with the counter-rotating lower unit and 1 season after I bought it. There doesn't appear to be any additional stress cracks or other signs of "unscrewing".