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 am finally purchased my Xbox S.

even the 512gb is too little. i do added External 2TB SSD on it.

games can now store outside.. but still facing little problem.

Boss, where did you buy your Series S from.

What about games etc?

Planning on getting one to play forza and fifa.

Many thanks.

Bought Series S and been using for one week. Bought gamepass ultimate from one of shopee seller. Most of the game i played is from previous gen console such as 360 and Xbox One so i directly install and play though external hard disk. The only game installed on internal drive is Forza Horizon 5. Never regret buying it coz i am just a casual console gamer, mainly game on PC.