So my question is, could I connect one of these types of routers to a LAN port on the ISPís router and effectively use it as an Access point?
Yes, you connect it to a LAN port.
.... Creating a faster Secure Wired/Wireless LAN for the shop
Connecting the guests in the way you suggest, will give them access to your shop network
You really need to put your shop network behind another firewall.
Line One:- Zen - DrayTek Vigor 2600VG
Line Two:- EntaNet - DrayTek Vigor 2600