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.
Awesome advice buddy.
Really appreciate it.
4k upscale is enough for me.
Not gonna do serious gaming.
Tempted with Switch, however Nintendo and its limited titles at times does affect my decision.
Good point on the physical disc. Something that may need to be put in consideration as i can see some selling used games here and there, does help to minimize the cost of owning the games too.