You may find Stephen Gandy's Cameraquest guide to be useful:
http://www.cameraquest.com/mguide.htm
As well as Andrew Nemeth's Leica FAQ page:
http://www.nemeng.com/leica/002be.shtml
1) Well, there's an obvious difference between the single & double stroke models, but perhaps less obvious are the fact that the single strokes are newer & incorporate several minor improvements that Leica introduced over the years into the M3. For example, if the single stroke is new enough, you can get the DoF indicators in the RF patch that were introduced w/the M2 (described in Mr. Gandy's page above). Having handled both, I prefer the single stroke mostly because it's more efficient; you can always use 2 strokes on a single stroke if you want to, but you can't advance w/1 stroke on a double stroke (though you can have the double stroke converted to single). Also, IIRC, the double stroke uses different parts that Leica no longer makes, so if the advance mechanism breaks, you have to replace it w/a single stroke, anyway.
However, some prefer the double strokes because of a perceived smoother advance & higher, even more old-school, build quality.
2) Yes. I think Leitz introduced the "modern" quick-load system in the KS-15(4) (a military version of the M2) back in 1966.
3) Be on the lookout for VF/RF separation, visible in its initial stages as golden "drops" around the edge of prism when looking into the VF from the front of the camera (towards the lens). This is discussed in Mr. Nemth's page above (towards the bottom). If the separation becomes total, the entire RF is shot & has to be replaced by 1 from a donor camera or re-cemented @ great cost by DAG, etc.
I am itching to get an M3 but before starting my hunt I wanted to ask you guys a couple of questions...
-Should I get a single stroke or double stroke? What's the difference?
-Is it possible to have DAG or someone else install the same film loading mechanism like the modern Ms (M6, M7, etc...)
-Anything else I should be looking for?
BTW, I want an M3 to shoot and not collect.
Thanks,