The Plan

Yes. The Plan is free to play on Steam, so there is nothing to buy to start playing.

The minimum requirements for The Plan are: Windows Vista, Intel Core i3, 1 GB RAM, 120 MB storage. It also runs on macOS. The recommended specs are in the System requirements section.

No. The Plan is a single-player game.

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

Yes. The Plan is the full base game. You need nothing else to play it.