If the game crashes at the start, right click on exe, start with resolution 640 x 480. Game will start and you can change the resolution in-game. Close game. Keep it on 640 x 480 for the start. Start the game, using settings from last start and the game starts with the resolution you changed before.

Does somebody know if the X: tension add-on is included in the X: BtF?

Yes, it is included in the version sold by GOG. It clearly says so on the game card:
https://www.gog.com/game/x_gold