I saw issue 57 of MagPi magazine, the official Raspberry Pi magazine, on the newsstand last week, and it included a hardware kit to build a voice assistant like Google Home or Amazon Echo. It is produced by Google AIY Projects which aims “to put AI into the maker toolkit, to help you solve real problems that matter to you and your communities.”
You just have to buy your own Raspberry Pi, and the SD card for storage, and a power supply if you don't have one handy. I got the Pi 3 Model B. A phone charger works well enough as a power supply, as long as it can put out a certain amount of current.
It was pretty fun to assemble, nothing tricky and no soldering. Initially, the button light did not work, but a few minutes of flipping the LED and jiggling connections fixed it. I would say even a kid of 10 could do the assembly. The trickier bit is in doing the authentication stuff and getting API keys, etc. If you have not done such a thing before, it's no big deal since the directions (in the physical magazine and the AIY website) lead you through it step by step.