Alright, here's my overall analysis. To anybody who knows Halo's lore, this should all be pretty standard.
The SPARTAN's armor is definitely a Mk-V. No debate to it.
Frankly, the helmet looks a tad ODST to me. However, in the picture above, the helmet has a few ODST-ish aspects to it as well. So, perhaps, this was the original version of the Mk-V helmet that was deemed too 'mean' looking?
Plus, you have to take into account that Halo: Reach takes place between a week and a day before the events of Halo: Combat Evolved. The Chief was wearing the latest and greatest gear, the Mk-V, so it'd make sense that all the other SPARTAN-II's were wearing it too.
Each soldier looks to me like they're wearing modded suits of Mk-V. The skull guy has what looks like an EVA helmet that has a painted visor. The sniper's wearing what looks like a Recon helmet. George is pretty much wearing what looks like a Mk-V version of CQB.
The helicopter things aren't precursors to the Pelican. Like Leatherneck said, it's possible their a Halo variant of the MH-6, which is commonly used as a Green Beret insertion vehicle, command post, or weapons platform.
The SPARTANs themselves are most likely SPARTAN-IIBs. If you consider John-117 and his SPARTAN-IIs as the A class, then these are probably the B class, or the second wave of SPARTAN-IIs. In Halo: Fall of Reach, the book mentions that Mendez will be in charge of training the SECOND wave of SPARTANs. Now, most people assumed that these were the SPARTAN-IIIs, but it may have been possible that he trained the second group of II's as well. Who knows? I have another theory listed below.
Now, I did note one discrepancy. The SPARTAN program was under the command of the UNSC Navy, meaning that all the SPARTANs were given Naval ranks and insignias. BUT, the FNG, the number 6, was called "Lieutenant".
I did come up with a little theory for this. It's very possible that the UNSC Marines (or hell, the Army) decided to create their own class of SPARTANs. This theory fits somewhat, as the Marine Corps. operates through the Navy and has to request resources through them. Having their own contingent of SPARTANs would remove this problem and they would have their own group.