The button rewind has to be held down on everyone I've used. I still use one picked up second hand in 1972. I bought it from an AP photographer right after she'd covered both the Republican and Democratic conventions in Miami Beach with it. She bought it from another guy I knew who used it during the Mexico City Olympics, but he bought it second hand at Browne's Photo Center where I shopped. The first owner had traded it in on a brand new M4.
The M2 was marketed in several minor variations: M2 with button rewind and no self timer, followed by the M2 and M2-S, both with lever rewind. The "S" had a sef timer. They then introduced a no self timer model again, designated (at least in the catalog) as the M2-X. Then there were a few years with no M2 models until a military model came out, with a civilian designation of M2-R, which had the M4 loading system. I still use my M2-R. You can't kill an M2!
For a few years Leitz offered a "rapid load" kit for the M-2 and M-3. You had to pull the spool slightly to reset the film counter on the M-3. They really were a royal pain to use. Instead of haphazarly poking the leader between prongs you had to align it and push it into a slot in the end of the spool. After a couiple of years the big mail order places closed them out for $12.50 Now, of course, they're worth a small fortune, especially if you still have the original box and instruction sheet.
At any rate, don't be hesitant about buying an M2 that's fifty years old, even if it saw heavy pro use. It'll still outlast you!
The original M2 listed for $200 in the early 60's, the M2-S was about $250, and when the M2-R came out it was sold together with a Dual-Range Summicron for $375. By that time the $5 hood had suddenly started selling for an outrageous $7 but you could still buy the leather neck strap with rubber pad for $2.98.
As for my M2-R? No collector in his right mind would want it the way it looks after forty years of ACTUAL USE, but I still have the pristine original box with matching serial number, instruction book, and a few of the Leitz brochures telling about what a great camera it is.