The 6by was the bmw m57 engine which was never very popular nor had a good reputation. I'd assume the 4by which is the only 180 yanmar thats available will be the m47 engine which was bulletproof in the cars but no idea how they fair as a marine engine.
Id make more enquiries what engine it is.
If its a 4lh then happy days they are a good engine if its a 4by I'd be a bit more cautious and do some homework to see what they are like & confirm what there based on.
Not saying they are bad just a bit of an unknown.