Okay, big long reply coming!
You'd be going from 21st century technology and working methods and end-results (infinitely reproducible and infinitely malleable digital negatives, electronic image storage,and inkjet prints,easy on-line created photo books) back to 1953 technology and working methods: a camera that forces you to remove the baseplate and to bottom-load each roll, meterless,and expensive film and development costs for every roll, single-copy negative storage,and rangefinder focusing, and a basically 90mm lens top limit with accurate framing and focusing.
While the Leica M3 is/was a beautifully made machine (I wanted one sooooo badly in the mid-1980's), it's basically optimized for the 35mm and 50mm lens lengths, and has a rather crude viewfinder system when it comes to the telephoto lens lengths...not very precise on the rare,longer lenses, which were basically the 90mm, and the rare,uncommon 135 which never really caught on. The 75mm lenses were rare too.
I dunno...the Leica, and other high-end rangefinders were interesting cameras, but not really as versatile as an SLR type camera. No macro capabilities with the M3...not worth a darn on closeups...not useful for lenses over 75mm in length...unable to focus high-speed tele lenses accurate at close ranges at wide f/stops...no zoom lenses...needs separate viewfinder if you do want to accurately frame longer-lens shots...atrociously slooooooooow X-synch speed of the Leica is pre-Korean-war slow.....no meter, so color slide exposures need a meter for good accuracy...B&W or color neg exposures can be estimated by experienced shooters well enough...not really worthwhile for most action shooting except with short lenses and zone focusing...has a rather narrow window where the camera's basically "1920's design ethos" produces optimal results....the Leica was designed to be a small,light,fast camera for capturing moments back in an era when a tripod-mounted,slow camera was the normal camera....the original Leica's design ethos was RADICAL AND NEW in the 1920's...its actual,closest modern camera ethos is exemplified in the smartphone camera: fast,easy-to-carry,light,ever-present,intuitive,one-lensed,easy to shoot at close to normal ranges,not good for distant subjects.
I dunno...I've worked a lot with the 28/2.8 and 35mm f/2 and 50mm f/2 lenses as well as the 85/2 Nikkor lenses...this is the basic area where the Leica shines: 28mm,35mm,50mm, and to a lesser extent,much less, the short tele range of 75mm or 90mm.
You'd be trading an exceptionally versatile,capable,modern camera, the 5D-II and a 70-200mm f/4 autofocusing zoom lens for a camera that premiered in 1953, and which is suited to far fewer types of assignments, suited to slower-paced,single-shot shooting, and which is fairly poor for action or distant subjects.
I get the Leica mystique, and the cameras and lenses are very nicely made. I'm not sure what you want to use the Leica "for". The 35mm film rangefinder camera lives on in the Voigtlander brand, and I bought a 35/50/75+body Voigtlander Bessa rangefinder system in 2001...it was nice, but not Leica-grade in body construction, but the lenses were sharp and crisp and small and light. But honestly, for me, the rangefinder way of working was not something I wanted to do, especially with 35mm film.
The new Leica M-series digital cameras appeal to me,however, just for the capture medium: raw, DNG digital on reusable media. You asked, "Am I crazy?". I dunno...are you? A 1953 designed Leica M3 and a 50mm lens is a far cry from a 21st-century Japanese-engineered Canon 5D-II digital single lens reflex camera. The two cameras are vastly different. Each one has strengths that the other one lacks.
