Let me ask you this, are you going to convert your current 50 or buy a different used 50? Also, did you buy your current 50 new?
I did a lot of research into the options you are asking about when we bought a new 50EX. Performance wise the two aircraft are the same, cost of operations will be the same.
If you go with the full conversion, the -40 engines and Collins Pro-line 21 upgrade cockpit package the modified 50 will have a slight edge avionics wise. However, you still have an old airframe, old wiring, old hydraulic lines, etc. Because of the later we chose to go with a new 50EX.
Sorry to say I have forgotten the down time for the conversion, but it is a rather long time. So if you are going to convert your present A/C you need to figure out what you are going to do about a replacement A/C during the downtime.
You also need to add in the cost of a new interior and paint (if needed, usually is) into the price and downtime for the conversion. However, if you by a used 50EX you may need to replace the interior and repaint that aircraft as well, so that cost would be the same either way you go.
As you are probably aware the Falcon 50EX is now out of production, however, Dassault still have two 50EX's not sold. At least they did at the time of the NBAA convention. I would definitely contact Dassault and see if they would make you a deal, they would like to move theses airplanes.
From my research the cost difference is not that much, especially if you by a late model used 50 to modify.
As of last month, well September, there are 4 used 50EXs on the market. Highest time 4,800 hours lowest 2,400 hours.
If you need any more information please PM me at your convenience.