Apple Watch with cellular

Cellular Carriers that support the Apple Watch, which models, and the radio frequencies each model uses:

https://www.apple.com/watch/cellular

You do NOT need to buy from your iPhone carrier, but they would need to support the Apple Watch on your iPhone plan.

Also note that cellular Apple Watches have regional radios, so do not buy in country X and expect to use it in country Y unless the above list of carriers and Apple Watch models says it is the same model Apple Watch.

Yes, you can always add it at a later date if you so choose.

Before you do that, make sure your carrier do support Watch cellular.