Nice find and it sounds as if you have a good sense o humour buying a McDonald.
They are a great little engine and we have the same model under the house waiting for it's turn to be bought back to life. At least now we have all the parts and some spares.
As for price it is always hard to put a figure on it. Sellers always go high and buyers tend to go low with prices. So make an offer and see what happens. If he is happy with your offer he will let it go if he is not happy with it he will come back at you with another price. So pay what you are comfortable paying. Out of curiousity what is the asking price?