The human body is a great blocker of wifi signals, so you might want to consider a wired rather than wireless setup. 
To avoid the need for high volume near the speakers, you might want to consider multiple speakers distributed around the shop. If so, I’d go for multiple One SL’s rather than two Fives. 

I agree with this for the most part, but I would go with Era 100s rather than the One SLs, which I don’t even see available on the Sonos website anymore .  The Era 100s is the more current model and has 2 tweeters giving you a stereo image from a single speaker, rather than mono. It’s a little more expensive, but should give you better sound and a bit more flexibility in adding additional speakers as needed in the future.

I would also recommend looking as Sonos for business.

https://www.sonos.com/en-us/sonos-pro

Hardwiring the Era range will also require the adaptor; the One/One SL incorporates it. I’m also not sure how critical stereo sound is for a shop? 

In a shop, I’d definitely be going with mono.

Good point.

I would not go with stereo pairs, but I would prefer a single speaker producing stereo audio over a mono speaker.

If the music is “background”, I still don’t see a need for stereo. A fairly even volume level across the shop floor is more critical. Price difference for One versus Era100+Ethernet adaptor is about $70 per speaker; if using 4 speakers, that’s a big price hike to go the Era route. 

Not to mention the monthly cost for ‘legal’ music use….

I’d look at the Sonos Amp powering six Sonos-Sonance architectural speakers in mono mode. You can put the Amp in a secure spot and wire it to Ethernet to eliminate the potential WiFi issues.

If six speakers isn’t enough for your space then a Sonos Port and an external multi-channel amplifier would be good. That opens up your possible passive speaker selection too.

https://www.sonos.com/en-us/shop/wireless-stereo-components

https://www.sonos.com/en-us/shop/architectural