I have about a million opinions and recommendations regarding iPads and communication... but #1 - if you spend the money on an Otterbox case, the iPad should be safe in your DC's hands. My dd has an iPad, and it has bounced down an entire flight of steps and landed on ceramic tile and been just fine (sadly, so did dd, but fortunately, she was fine, too).
As to software, I have had an opportunity to attend several training sessions for different apps, so it depends on cost, needs and funding. I'd be happy to pitch in, though.
If you choose an iPad as your device of choice, it is best if communication is its only purpose - no games, no other apps, just communication. Especially in cognitively "intact" children, if there are games to play, the likelihood that they will want to take the time to communicate if they could be playing instead is less than stellar. If you are seeking funding from an outside agency (whether governmental or private), they may be reluctant to pay for an iPad because it can be used for so much more... but they will pay 10x the cost for a "dedicated" device.
Makes no sense to me either...
Aside from that, your DC's motor control is very important. Finding an app (if the iPad is the way you want to go) that can adjust the size of buttons and number on the screen is crucial. You can start with 2 or 4, then, as control improves, you can reduce the size of the buttons (with blank buttons in between) and then the number of active buttons on the screen. There are overlays for the iPad available to put a tactile barrier between buttons, but they do not necessarily fit with a protective case. I can show you how to make a divider at home that is both easy and a universal fit.
Lots to consider, but I'll answer what I can from the minimal experience and exposure I've had.