Hi Jim,
You seem to have a good, thought out attitude for this...which is very good, many ppl will just want "the biggest he can get", which is definitely the wrong approach in this case.
Let's look at some facts/figures:
A Nordhavn 46 around 1990-1995 is just under 500k, while a N43 2004 (as seen on a listing website) is around 700k?
Are these the figures you are seeing in your firm buying contacts?
That in itself is a BIG difference... HOWEVER, a 15yo boat will be most probably a big maintenance ticket... Engines, pumps, generators, exhausts, everything tends to deteriorate, and when you talk 15yrs down the line, even a top-notch maintained vessel will have many problems...
A brand new boat will have its kinks to be worked out (so not exactly "pick-up and cross the ocean"), but it will have warranty on all major components. This warranty is not only from Nordhavn, but think about the generators, engines, aircon, and all other systems.
Hourly techs just to figure out what is wrong with this stuff can cost hundreds of dollars just for a diagnosis in some cases.
10-15% (lets assume 50,000) is probably enough to get a 15yro boat in "useable" condition, but probably wont be enough to swap out any major components that need rebuilding/replacing.
A new aircon setup will eat a considerable part of that sum, as will a generator swap/revamp, as well as an engine rebuild if required.
Maintenance on a new boat will always be less than on a used boat in the long run, especially since your "used" boat includes probably no warranty, and the new one will include full warranties (which can range from 1 to 5yrs or more depending on the equipment/extended warranties).
What about fuel? Both of the boats mentioned carry around 1000gals of fuel, which can cost you about $2000 or so to fuelup. Although they will give you a lot of range (the N46 has even smaller engines, so running hours on one tank will be more I suppose), you will still be fueling up rather often I suppose, so that needs to be factored in.
Why don't you investigate a bit more about the two boats you are considering (is the used one well maintained? have the engines/generators been overhauled? how many hours do they have? When is their next overhaul due?, etc)...
These nordhavns have the john deere engines, which supposedly run for over 50,000 or so without an overhaul, but there is probably a good amount of maintenance involved.
Cost for oil and spares should also be accounted...I really don't have enough experience to give you concrete figures, but maybe something 2 yrs old or so, or maybe a bit older would be better? (Things tend to have been fixed under warranty, as opposed to things being overlooked because they were quite expensive and were still "running")
Others will probably have more details, but do look around, maybe ask around to some current captains of similar boats? Captains or Captain-Owners will probably be able to give you concrete numbers, and maybe they will disprove everything I say