Rifles are too light for spartans (imho). I'd stay with Arc Troopers:

I'm going to be painting my Khurashan mid tech humans at some stage, which have a nice spartan feel to them:

Also with regards to Critical Mass Games, their augmented ARC drop Troopers will make perfect 'prototype' suits (as seen in the Halo Legends DVD). The drop trooper mercs would make for some nice spartan armour loadout options too.
GZG NI troops have that powered armoured MK V look, and the USNC 15mm troopers have a definate EVA helemt armour loadout look which could work as well.

Both are later sculpts and you shouldn't just dismiss GZ, they've been going for a long time and the future's looking very bright for them with the new products coming out this year.
ACP Games:
http://acpgames.com/2011/11/mercs/mercsoo1/Have produced a six man merc team, (a very 'NOBLE' looking team, if ever I saw ONE just within your 'REACH' if you know what I mean,

I'll dig out the link).
Plenty out there, good luck
The Commander