Reasons to get Series S:

You don't mind / don't have the means to display at 4K resolution
You will be primarily using it for Gamepass and digital games
Cheaper pricing / better availability


Reasons to get Series X:

You want your games to look at its possible best, with 4K resolution and support for ray tracing
You still use / prefer discs. Also, you can play 4K Ultra Blurays too
The built in SSD is bigger in size (1Tb vs 512GB on Series S)

As for your other questions:

1. I would advise to get it from trusted sellers like Impulse, since they will be able to help with warranty.
2. I like using US account, since there are some differences in title availability by region. That was in the Xbox 360 days though. The currency that you will be using also depends on which region you use
3. Personally I prefer using Series X, because I still have a lot of physical discs from the OG Xbox, Xbox 360 and Xbox One. But if you have no intention to buy physical discs, and don't really mind a drop in performance / visual quality, a Series S is a very good purchase, since it can run the same games as Series X.

I use the official Xbox rechargeable battery. Can just connect the controller to the Xbox while playing. But quite a few of us here use rechargeable batteries like Eneloop. Beats having an internal battery that can't be easily replaced if it dies.