The cell that you have stopped using because you replaced it with a newer one, now you can use it again transforming it into an intelligent speaker.
Not really. You can use all those compatible with "Google Assistant", which are those that have installed "Android Marshmallow (6.0)" onwards, with more than 1.5 GB of RAM and at least have a screen with HD resolution.
It is very easy to verify this compatibility. You must press and hold the Home button for a few seconds and say "Ok, Google". In case the device is compatible, the virtual assistant will automatically activate with which you can start to issue commands.
Yes. A traditional speaker, which can be linked to the cell phone by means of bluetooth or external cable. The cable connection is recommended because many times the wireless speakers are disconnected if a certain amount of time passes without being used.
Once the compatibility with the Google assistant has been detected, you must enter the Settings menu of the device to the Settings option. Then go to Google> Search, Wizard and Voice> Voice and activate the "Voice Match" option there (talk to the Google assistant to perform tasks).
In this way the assistant will be attentive to the voice commands dictated by the user at any time, even when the screen is off. Then you have to link the cell phone to the speaker.
Ideally, keep the speaker and device plugged in permanently, to ensure that both devices are ready to operate when listening to voice commands. No password should be placed on the mobile device. By the way, this smart home speaker does not have the same precision as smart speakers, like Google Home, but they work fine anyway.