I didn't start using a rangefinder camera with Leica. My first one was a Voigtlander Vitessa L, a pocketable little gem that served me well for what I was doing at the time. But my usage needs changed, and I switched to SLRs for a long time. Eventually I purchased another fixed lens rangefinder camera, and several more after that for mainly occasional use.
Eventually I wanted to get serious with the rangefinder, wanting to be able to switch lenses. So that progression went Contax IIa, Nikon S2, several FSU cameras, Yashica, Canon, and Nicca. Even Leidolf. Anything but a Leica. I wanted to see if I could produce just as good an image as someone with a Leica. And it can be done. Not that I felt Leica users were snobs since there are plenty of examples of working photographers who used them for journalism and such.
What it all came down to one day was the lens mount. I always felt that Leica's were too expensive for what you got, but when it comes to that M mount, nothing beats it. Like the first Nikkormat I bought, once I used that bayonet mount there was no looking back.
So to answer #1, for me it was the usability of the system that attracted me to the M line. I shopped around too, keeping my cost of joining the ranks by buying used equipment. As for my infatuation with the IIIf, that has more to do with giving up on all the LTM pretenders out there that either didn't have as much to offer, or just weren't as well built as the Leica.
As to question #2, I don't have any experience there. To save on cost, probably a Panasonic equivalent is in the future.
PF