Are you certain you want the M3? It's a great camera--as great a camera as Leica ever made--but I ask because of the viewfinder. If you want to keep using your 40mm lens, you'll need an auxiliary finder. And if you wear glasses, the M3 finder is tough to use--very hard to get your eye close enough to the finder to see the 50mm frame line. Also, glasses wearers often need to add a soft covering to the metal viewfinder ring so their glasses won't get scratched on the metal. And think also about metering. If you've been using the Voigtlander, you've probably become accustomed to an in-camera meter and faster film loading--are you okay with eliminating those two features?
If you're still set on an M3, what's most important (to me, anyway) is how recently it's been serviced (and where) and how bright the finder is. If you think the viewfinder, lack of meter and removable spool will become issues, consider something with more frame lines and better eye relief. I realize an M6 (.72 or .58, depending on the type of finder that's right for you) will probably cost twice as much as an M3, but if you're looking for a camera to grow old with, then it may not be such a steep investment. Good luck with your purchase.