If you just need one for that, the cheapest Echo Dot model. If you care about a nice Bluetooth speaker aspect then get an Echo. If you wanted a screen for whatever reason then an Echo Show. But Echo Dot is the cheapest model. Oldest version you can get is fine.
Leaving the microphone muted should prevent packets from going to the cloud but I can't find definitive information either way. They process the wake word locally so answering to say "the device is muted" (if they do, mine don't, but I've heard some do) doesn't necessarily imply it's sending anything to the cloud.