Loot Box

Yes. Loot Box is free to play on Steam, so there is nothing to buy to start playing.

The minimum requirements for Loot Box are: Windows 7, Intel HD Graphics, 2 GB RAM, 500 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Loot Box is a single-player game.

Mostly, yes. Valve rates Loot Box as Playable on the Steam Deck: it runs, but may need some manual setup.

Yes. Loot Box is the full base game. You need nothing else to play it.