Check the hour meter on the engine's smartcraft gauge yourself to confirm the 420 hours.
Transom: Have a friend watch where the engine mounts to the transom. Stand on the lower unit and bounce up and down. There should be ZERO flex or movement. Any movement indicates transom rot, which is a deal-breaker. Also look for any hairline cracks in the gel coat around the transom.
Deck: Walk on every square inch of the floor and the front deck. Feel for any soft, spongy, or flexible areas. This indicates rotten wood underneath the fiberglass and is a very expensive repair.