Welcome.
To address your questions:
1. Maintain it like any AR-pattern rifle. There are thousands of resources in print and on the web about AR maintenance.
2. Run it wet.
3. A little Googling should have revealed that Bushmaster shipped a ProMag 9mm mag with these guns. The mags are still available from some sources, including Midway USA.
4. In the future its a good idea to never buy a gun without first determining if parts, especially mags, are still available. There's sometimes a good reason why a used and/or discontinued firearm is cheap: it's been orphaned by the manufacturer, as the Carbon 15 appears to have been.