Dude, I'm one of the biggest synthheads on the planet. I'm friends with a LOT of synthheads. Not one of them gets wistful about the M1 because its architecture and feel were duplicated across a five year period (at least!) of synthesizers. It's just another boring ROMPLER not half as exciting as a JD800 or Korg Wavestation was. Heck, the stupid K5 is more exciting than the M1.
The M1 is 'classic' in the way that gated drum sounds or hair metal bands were... it screams instant 'cheese'. Avoid at all costs. There is nothing that the M1 that a dozen or so other synths do TEN TIMES BETTER. At the time it came out, however, it combined sequencing, ROM based sounds and multieffects into a single unit and thus was very impressive. Nobody really works that way anymore... that time has passed.
Anyway, almost all ROMPLERS are superfluous nowadays because samplers are so darn cheap. All a ROMPLER is is a sampler without the ability to sample that has to draw from a small 2-6 mb pool of waveforms. Yawn.
If you want something from that era, get a darn Kurzweil K2000 fully expanded and rock on. That is still a great keyboard.
Keyboards that get synthheads drooling are usually way cooler than the M1, and often way older. The Les Paul of synths is the Minimoog. The Fender Strat of synths is the Prophet 5 (my personal favorite of which I own a mint, MIDI'd version). Synths that get synth geeks excited are typically older analogs like the Jupiter 8, Juno 60, Arp Odyssey, or Oberheim OBXa; digital synths that make us excited are usually well-spec'd VA synths that duplicate the vintage analogs, or the occasional weird synth (Waldorf XT) or something that uses a different type of synthesis like additive or FM.
Korg M1... HAHAHHAHA! The Korg Wavestation owns over those and they are a lot cheaper. Uses similar internal architecture and the EX version should have all the ROM snippets in there... and it is cheaper to boot. Vector synthesis... yummy.