The game comes directly from PSN Store, you will be able to go online, get updates, etc.
After the setup process, the game will be available to launch on your own account, just as any other game or app you purchased directly in PSN Store.
The Mercenary is a special class loadout playable from start through this DLC only. The Mercenary has dual-wield as a starting ability and is geared up with a classic Katana and the shorter Wakizashi blades to explain it in real-world terms. Simply put, it'll let you experience the world of BIOMUTANT as classic samurai looking warrior from the start.