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A Mercury Laser II (also sold as a Quicksilver Lightspeed) prop in 20" pitch should be perfect for that engine on a Montauk.

But I would not buy that boat until you go for a sea trial, preferably with someone along who knows something about Mercs, Either the seller is a complete amatuer, and knows about engines in general, (that's a bunch of baloney he was giving you) or he is covering up an angine that may need serious work. The symptoms may sound like the high speed stator is out. If you buy the boat, expect to spend $1000-1500 getting the engine in shape, or even having to buy a different engine.