Can I connect the Mitras LX7, GHL Doser and ProfiLux to the cloud? Do I need to buy anything extra for that?

Cloud access, yes. Extra accessories needed for cloud access, no.
GHL Doser, ProfiLux 3 and 4 are equipped with Wi-Fi and/or LAN that allows you to connect it to an existing network. You can also connect all devices to the myGHL cloud for remote access to your device. All mentioned devices are cloud ready right out of the box.