Would I also have to turn off DHCP in my FritzBox router, so that it doesn't conflict with DHCP in a wifi router?
Make sure before you do anything insure that the Fritzbox is on a static ip once dhcp is turned off. use that ip as the gateway ip for the APs. you may find it easy to have both connected to a pc/mac at the same time to make sure you have configured both units. Until we know what mesh or aps you are getting we are sorta in the dark.
also you can have the fritzbox wireless available, but thats after you have set the mesh ap/aps up.