IME substrate isn't all that important for shellies, at least to them. They'd rather dig down to the bottom of the tank. I ended up with huge mounds of sand on either side of the shell pit. But its fun to watch them dig. The males will pick up sand in their mouths, scurry to the outskirts of the pit, and dump.
You shouldn't need much sand. 1.5" is enough really as shells are far more important.
Unless your water is very soft, you shouldn't need the eco complete either. I'd personally save money by going with pool filter or play sand. Even with softer water I'd likely go the crushed coral route. Your choice though!