I'm currently running a Moto G3 with Marshmallow and Google Now. My Google Now feed has been bombarded with articles about how I'm going to get Google Assistant for over five months now and as an Australian I thought it would have happened already. Despite what you may read, it isn't going to happen automatically and this article describes how to make it happen.
Firstly you should try Google Assistant via Google Allo to determine if you want it in the first place. Personally I ask AI systems "who shot first" to determine their quality. Google Assistant's comments about Chewbacca impressed me enough to make the switch.
A word of warning you won't find on the web: this process will destroy your launcher configuration and means you'll have to set up all your icons and widgets again. If you've ever moved from Lollipop to Marshmallow this is something you have become accustomed to anyway.
Are you ready to do this? Let's go!
Step 1: Make your phone American (it's only temporary). Go to settings then "Language & input".
Step 2: Kill Google. In settings go to Apps.
Step 3: Configure Google Assistant and rebuild the universe. Go back to your home screen and you should notice it has been destroyed. Swipe right to where Google Now was and it will ask to configure Google Assistant. Do this and rebuild your launcher icons and what not.
Step 4: Get the hell out of Dodge. Remember to go back to settings, "Language & input" and change your language back to English (Australia) or whatever.
You'll have to keep your language as English (United States) or Google Assistant will stop working.