yeah, it's the same Maelstrom engine as the one you use for ERAS or all the other games. Just make sure to update your Maelstrom engine install to the latest version.
So here's the latest update?
https://chezjfrey.itch.io/gentlemen-of-fortune-maelstrom-engine/devlog/734002/ma...
If I bought it once (when I wanted to install AoP2 GOF 2.0), shall I buy it once again?
yeah, thata's the latest update but you can just download it from here: https://chezjfrey.itch.io/gentlemen-of-fortune-maelstrom-engine
It will automatically download the latest version and as long as you are logged in you won't see the buy button but rather the download button on the top of the screen. There is no need to buy it again as all updates are included.
you have to click on the individual things like the ship or character picture to bring up more options. There are a ton of different things to choose from. What you did is basically clicking through the individual storylines there are (which are quite a few + individual prologues for some characters in freeplay). You can check out the wiki for quest walkthroughs and other stuff here: https://www.piratesahoy.net/wiki/new-horizons/