I'm not familiar with Mercury-Hermes, but I can, perhaps, shed some light on the difference between the standard Saiga and the Arsenal version.
The Saiga is a Russian-made AK variant. Unlike many other AK-variants, it is made and assembled entirely in Russia. In order to be importable under 925(d)(3), however, it must be set up in a sporting configuration which means it can't have certain "assault weapon" features like a pistol grip, flash suppressor, or bayonet lug. Hi-capacity magazines are a bit of a legal gray area, but standard Saigas come with 10-round mags will not function properly with standard AK magazines.
It is fairly common to do a "conversion" on a Saiga in which the "evil" features of an AK are added. In order to do so legally, however, a certain number of specific parts must be replaced with U.S. made ones so as to comply with 922(r).
The Arsenal is basically a Saiga that has already had the 922(r) conversion done. A standard Saiga can be converted by a home gunsmith to a "semi-AK" configuration fairly easily and inexpensively, but in order to add all the "real-AK" features (except full-auto capability of course) you probably wouldn't be saving that much over the Arsenal.
It really depends on what you want. I converted my Saiga .223 over to a fixed buttstock and pistol grip (which also requires replacement of the fire control group) for $80 worth of parts and my time. I did not bother to install a bullet guide (necessary to use real AK mags) because aftermarket Surefire Saiga mags are not that much more expensive than "real" .223 AK mags. If my gun was in 7.62x39 or 5.45x39, I would have done the bullet guide as well.
