If all you are looking for is Bluetooth, I’d get a Bluetooth only speakers.  Sonos is a WiFi based smart speaker system first and foremost, Bluetooth is kind of an afterthought. 

Define ‘Wifi’ in your question, please? The question seems to imply you mean an outside connection to the ‘internet’, but it seems you have a local LAN, based on the ‘Ethernet’ statement. Can we get more clarity?

For what it’s worth, as stated by @jgatie , Sonos is an internet based system, and certainly requires access to Sonos servers to set it up, initially. You might be able to use it without access to the outside world after it is set up, but as was put, you’d be better off with straight Bluetooth speakers, if the system doesn’t have access to ‘the internet’.