iWatch 7 GPS + Cellular

Doubtful. To avoid having any problems, I suggest you purchase the watch in the country where you intend to use it. It will simplify any warranty or support should you ever need it. Also, make sure that you have an eligible cellular service plan with a supported carrier. Your iPhone and Apple Watch must use the same carrier, and you must be within your carrier's network when you set up cellular on your watch. For more info, see:

Set up cellular on Apple Watch - Apple Support

So how much will you save when you buy said watch in the US and find out it not only doesn't work in Brazil, but you can't get service or return it in Brazil? It could cost you SO MUCH more money than you just buying the watch in Brazil, where you know it will work. It will work as a GPS only watch however.

To be clear, a US purchased Apple Watch is designed to work in the US, Canada, Mexico and Puerto Rico. That's it. Of course its your money to waste.

You're welcome.