It's definitely the M5 port mount; as I was originally upgrading from my Nexus D70 system, I chose to order an M5 mount instead of the M6 to accommodate the ports I already owned, and had the benefit of eliminating another potential leak point (M6-M5 adapter). I also looked into the lens compatibility, and found that all lenses I was either using or ever likely to use with the D300 would easily fit with the marginally slimmer M5. For example, I currently tend to use Nikkor 12-24mm for W/A and Micro-Nikkor 70-180mm for macro, but it would just as easily fit Nikkor 10mm fisheye or Micro-Nikkor 105mm VR.
Yes, I'd be willing to sell only the D300, but obviously with mutually-compatible ports it makes sense for me to try to sell both together, and I'd prefer to see the D70 also getting used instead of sitting idly in storage. I'd sell the D300 setup alone for £1,850 if you're interested. If you also want a wide-angle lens I'd consider parting with my Nikkor 12-24mm, but until now I'd anticipated holding onto it. Bear in mind also that it ships from UK, as from your profile it looks as though you're in US.
I've been diving at Isla de la Plata for a local manta ray project (http://mantasecuador.org/) based in Puerto López a little way South of Manta, and we never see sharks on any of the dives. Apparently there used to be sightings of reef sharks frequently, but no longer. The smaller fishery in Puerto López also catches some sharks, but not in the same numbers caught in from Manta, and they're usually bringing in small hammerheads, and some threshers. What striking is how tiny all the sharks landed are, well below breeding age, which obviously rings alarm bells for sustainability.