i need 2 pokemon to complete the pokedex finnons evolve form and tangagroth
Good you have a completed dex, you should go see prof.oak in the town with the Legendary statue (hasn't played in months) he might give you something worth getting.
I don't know I just remember that when you completed the hoenn dex you got a jotho starter so mabe you get something this time too.
you get upgrade for ploygon z
No you get that for completing the regional pokedex. Also its the evolution item for Porygon 2. Go to the Beach Resort place by the great marsh in one of the houses is a man who gives you a diploma talk to him, he might give you something.