Watermelon

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

The minimum requirements for Watermelon are: Windows 7, 128 MB RAM, 150 MB storage. It runs on Windows only. The recommended specs are in the System requirements section.

No. Watermelon is a single-player game.

Yes. Valve rates Watermelon as Verified on the Steam Deck: it runs well out of the box.

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