The one thing that has been over looked is this, you are buying something that is as yet unseen, its a second hand engine, you need to see it, and more importantly see it running, I would not pay a penny for a second hand item until then.Once they have removed your old engine then they have you by the short and currlies! Where about in the country are you, there is a massive choice of very good yards that have a good stock of second hand engines with very good reputations that will show you and engine running, give you written quotes and even give you a guarantee!