Yeah, unfortunately the Ardex stuff isn't cheap.
To answer your question, yes it is possible to grind 1/3" off the concrete slab. It may take a long time, depending on the equipment you are able to rent, but it can be done.
Rather than grinding down the high spots, my suggestion is to pour a self-leveling topping over the entire slab and then put the epoxy coating on that. Something like Ardex K 500 would work.
I've used a similar product (Ardex K15) to level concrete so that tile could be installed in a...