When buying a used 170 MONTAUK you will likely be buying the boat with its original engine. The engine used on the 170 MONTAUK varied over the years, from 60-HP FOURSTROKE, 90-HP two-stroke-power-cycle, 90-HP FOURSTROKE. The engine and its condition are very important factors in determining the value in a used 170 MONTAUK.
WIth regard to Mercury 90-HP FOURSTROKE engines, there are FOUR very different models used.
Prior to c.2006 the 90-HP four-stroke-power-cycle or 90 FOURSTROKE engine could have been a Yamaha powerhead engine with carburetors or a Yamaha power head engine with simple electrically control fuel-air induction (often called "injection" somewhat inappropriately) and called a Yama-Merc.
After c.2006 the 90 FOURSTROKE engine would more likely be a Mercury in-line four-cylinder 1.7-liter four-stroke-power-cycle engine based on the VERADO four-cylinder engine block without forced air induction, an engine often discussed under the nickname VERADITO, as in a Spanish sobriquet for "little VERADO." This engine had an unusually large domed cowling for a 90-HP engine. More on the VERADITO at
http://continuouswave.com/ubb/Forum4/HTML/005118.htmlIn c.2014 Mercury replaced the VERADITO with a new 2.1-liter 90 FOURSTOKE. I would expect this version of the 90 FOURSTROKE to only be found on boats made more recently, perhaps just in the past five seven years.
For a very good summary of the FOUR different versions of 90 FOURSTROKE engine from Mercury, see
A Brief History of the Mercury 90-HP FOURSTROKEviewtopic.php?t=2405Also, a careful reading of the REFERENCE section article on the 170 MONTAUK will give you greater insight on the 170 MONTAUK and some variations:
http://continuouswave.com/whaler/reference/170Montauk/For the latest 170 MONTAUK II models, read this timely discussion that accompanied their introduction:
Boston Whaler Introduces the 2018 170 MONTAUKviewtopic.php?f=4&t=2588