Is it possible to get different Ghosts?

According to the DataMining, there are a handful of shells in the game, whether or not we can use them all remains to be seen.

That being said, the one confirmed and ingame extra shell for now is the Frontier Shell, which came with the special editions of the game.

Shells are purely cosmetic, like ships.

In my version of Destiny (Taken King, just got it for Christmas 2015), you can buy Ghost shells from the Speaker, if you are high enough level.

Also, it is not just a hat, a higher level Ghost shell adds (I believe) defense, so it increases your light.