I never heard of using the Celestron wifi-module with INDI. I always read about the wifi-module combined with a kind of SkySafari planetarium called StarryNight.
I think you can buy a NexStar+ hand control separately. But in any case I think you can connect the stellarmate through the RS232 connector on the old HC.
You can also have a look at the indi_celestron_aux driver. It is in development. I have no experience with it but seems it can control the Celestron mounts through their AUX port. There is a long thread in this forum about it.