I have BT Homehub 5

Looking to get a wifi extender as house is long but not sure to get a dual band or this new ac band

Should I buy Netgear WN300 or BT 600 dual or BT 1200 dual and AC band?

Thanks

If you don't have any devices that use the ac protocol (it's not a band by the way) then there is little point getting an ac capable extender, other than for future proofing. The 802.11ac standard uses the 5GHz band that any dual band extender will also use. It is just a different method of transmitting and receiving data within that band to the other variants of the 802.11 family of WiFi standards.