Check the dashboard for the latest offers, they often offer the game pass, game pass ultimate or xbox live gold for around £1 for a month - just make sure you turn auto renew off, or you will be charged the full price the next month.
Xbox Live is usually £6.99 a month - allows you to play online with friends, chat with friends, exclusive offers, free games to download every month to keep, even after your live runs out.
Xbox Game Pass is usually £7.99 a month - Access to over 100 games, updated all the time, member discounts, available until your pass runs out.
Xbox Game Pass Ultimate £10.99 a month - includes both Live and Pass.
@Sunbelievable she should still have all her progress saved to her system or external drive, she just won't be able to play them anymore. Once you purchase the game, if you haven't deleted the saved data from your device, you should be able to jump back in from where you left off.
I much prefer a disk and agree with @ShadierThanaPalmTree that games often drop massively in price if you get them a little later and will be available secondhand (sometimes new) from CEX or Game, even Facebook Marketplace or local Facebook groups. You may want to consider new if it comes with a freebie though, as most are claim once only, and the original owner would have usually claimed it, unless you are lucky!
With disks you can buy cheaper and sell them once you are done, getting some money back. When I'm able to and choose to get the new Xbox, I 100% want the series X, as it is the one with the disk drive, at least I'll have the option. I know it is more expensive, but it will be worth it for me in the long run.