The title says XBox, but also consider the PS4. It will come down to what their friends have most of the time, which may be why you have said XBox.
As for games, is their anything they particularly want, again, maybe because of their friends, and then there may be offers on with particular games and console. Some games are only on specific consoles. The driving game Gran Turismo is only on Playstation, and the driving game Forza Motorsport is only on XBox. There are other examples for each console. Most of the bigger games are on both consoles, but maybe not be able to played with a friend (online) with the same game, on a different console.
You may already be aware, but you only get the most out of some games with an internet connection, and the internet will be needed for game and console updates too. I think the PS4 and XBox both have built in WiFi, so no need for wired connections. Unless you don't have WiFi of course.
Most games need to be updated the first time they play them, and will continue to recieve updates, either fixing bugs, or adding extra content. Some extra content may have to be paid for too. Depending on what games they get into, and if they want to play with their friends, there may also be a subscription with Microsoft (XBox) or Sony (PS4) needed to play online games.
Video games can be a bit more complicated and expensive than it used to be. That said, for the time being at least, lots of very cheap used games can be had to bulk up a library. I say for the time being, as there are a few physical games shops left trading in used games before the shift to game downloads. I have had every Playstation, but didn't get the PS4 until a few years in, and there were a lot of cheap games available S/H.
If you are aware of all of that, then I was just trying to help.