If you are interested in the boat and think its worth the money I would follow up with the shop that does the work. Indmar goes into a limp mode when you loose the impeller or the engine gets hot. 180 isn't that bad. Its when the needle is burried and the and engine starts smoking. If you are serious about the boat I would go with the guy to the service shop and talk with the service manager about possible damage. I would make the owner get a full service report and diagnostic prior to purchase. If the guy selling the boat really wants to sell the boat and sure there is no damage, he will do it. I have a buddy that is always burning up impellars (he's kind of an idiot) and runs the boat hot all the time. Remember its just a smalblock 350. These are almost bullet proof. But as mentioned before the exhaust is the weak spot. Pulling it out to inspect it should be fairly easy. Just some hose clamps. The damage would be fairly evident, glass blistered, cracks, ect.
Again if the seller wants to sell it hell jump through a few hoops for you.