I had an M-3, then traded up to a D-152.
(1) Unless the foldback kit has already been installed, the percussion will not sound as good as a B-3;
(2) It is not an "updated" version - they were out at the same time, and the M-3 typically known as a "Baby B". The M-100 is an updated version - I've never played one, but the L-100 had some presets, reverb, and an integrated start-up switch. The M-100 was supposedly better than the L-100 (which was used to record Green Onions);
(3) The M-3 has an "octave minus one" pedalboard - almost useless. also stubby plastic pedals, rther than the "board" pedals of the larger models;
(4) The built-in amp and speaker really add to the weight. Two guys dan carry it, but it's not pretty;
(5) Extra cool item - there's a mono rca female plug on the back for an external audio in - it was for hooking up a mono record player back in the day, but an electric guitar sounds real sweet through all that tube circuitry (a whopping 10 watts, if I remember correctly).
Even without a Leslie, it's still a very musical instrument, solid as a rock, and a lot of fun. I only ditched mine 'cos I needed a full classical pedalboard, and didn't have room to keep it after I moved the D-152 in. $350 is a good price - I only paid $100 for mine, but by the time I paid for help moving it and repairs, I was up to about $450. I believe going rate for one in good condition is $4-600, so you should get your investment back out of it if you decide not to keep it.
Good luck. don't forget to oil...
Daf