All modern ones are tack sharp with good contrast. The vintage one have less contrast. An important factor is to have one that is mechanically sound, especially the older ones. Shooting a test roll is not possible with virtual second hand acquisition so it's always a bet...
PS: yes handling is like the icing on the cake, and they are very different in handling. I like the "no hood version with tab" the most, I got the chrome version wich is heavier, I use a canadian plastic hood, I like the handling of the combo a lot. Also The collapsible cron has a nice compact feel, but not so compact compared to the elmar, so get a cron plus an elmar!
I may be off subject but I think that if you want a leica superb performer nowadays, you would probably better invest in the new 50mm summarit wich is with no doubt a superb lens capable of blowing away when use with skills (discretion assured)!