USG20w or 40w? Which Wireless Extender for a Warehouse and Barcode Scanners?

Hey guys, One of my customers Is expanding their warehouse and I need to take a decision on changing their current zyxel router, thus need a little help in taking a decision. I was thinking of offering them the USG20w I’m using in my home office and getting the new USG40w. On another note here’s the customer’s setup: The head office is in Montreal with 5 users, one of them Is a Motorola MC55 handheld scanner for receiving inventory (We’re currently using a NetGear well rated consumer router as access point with an older generation ZyXel wired router). The handheld drops wireless while scanning barcodes outside of the warehouse in the 40 foot containers, this causes havoc and needs to be addressed. We also have the Florida office which is running off of a ZyXel 2WG, has 2 wired users, 2 wireless from which one is a barcode scanner, this office is connected to the Montreal office via VPN. A software called Fishbowl Inventory runs on the server in Montreal where clients from Florida connect to (one or two users at a time along with a barcode scanner). Although only the connections to this particular software freezes regularly while VPN connections is still up, we’re wondering if current hardware might be the cause. In summary, would the USG20w be a good option for the Montreal office knowing we have a few users connecting to it from a remote location along with a few locally? The Montreal office is about 60 x 60feet, when a container is received outside we need to take in consideration another 40feet, what type of wireless repeater would you recommend? Sorry for the long post, thank you in advance.