The Saiga-12 is indeed a semi-automatic shotgun. In the "sporting" configuration, it's imported from Russia. If you want to use high capacity magazines, and it's legal to do so in your local, you need to "modify" the shotgun so it complies with 922(r). You can have no more than 10 foreign parts on the shotgun to use high capacity magazines.
The high capacity magazines available are 8, 10, 12, 20 round drums.
Your understanding of the gas plug is incorrect. The gas plug will meter the gas going into the gas block. There are 2 settings on the stock gas plug. Setting 1 is for high brass rounds, such as slugs and buckshot. It will decrease the amount of gas going into the operating system. Setting 2 is for "low brass" rounds like birdshot. It allows MAXIMUM gas into the operating system.
The number of modifications you can do to a Saiga are pretty limitless. There are new gas plugs available that are self regulating, as well as any number of pistol grip configurations you can put on a Saiga.
The cons: They're made in Russia and won't necessarily run out of the box 100%. You'll have to be patient and confident with either doing some simple tasks, such as polishing the bolt, and bolt carrier, and maybe even working the gas ports to deliver the gas needed for running "low brass" bulk birdshot.
Or be prepared to send the shotgun to a custom house that will do the work for you, for a charge.
You can spend as much on a Saiga as you want to, so buyer beware!!