Your thoughts on a flash
Not sure about the price since I don't own or use Sigma flashes, only Sony.
As for wireless triggering, the flash would need to be compatible with Sony's protocol, only way to find out is to test it or hope another owner of that model chimes in.
You could use the triggering cable Sony offers that mounts on the hotshoe and plugs into the flash, but that means the flash needs the port for that.
There is also the option of using a radio frequency trigger with it. That should by pass any problems if it doesn't fire wirelessly natively. The flash will think it's on the camera and fire.