Mark, the purchase of a 115 HP Mercury to match your pristine boat makes alot of sense. Is not your boat rated for a 120 HP motor? With a 115 HP Mercury over the 60 HP Johnson, you should be adding 10 to 15 MPH. That is alot of fun factor added. Your next to new 60 HP Johnson should command a premium price and the price difference between the 60 and 115 will not be that great. My best move was to switch from an 80 HP to 115 HP on the V-153 back in 1987 - added 10 MPH.
I would look for a mint 115 HP Mercury for your boat first. My 1988 115 Merc is about 1/3 the price of the remanufactored Merc. You may have to search around the country for the best. Once in awhile, a new older Merc is for sale, and the price may be less than the $4,450 of the remanufactored one.
Hay, one can justify their hobby for several reasons: one does not have alot of hobbies, one is conservative in their spending habits, one works hard to earn a living, the purchase will hold its value, and the purchase does not need to be financed. Good luck in your search. Use a search engine for national exposure.