The short answer is "No." All of the tanks are unlocked from the start, and you don't need to do anything in order to get them. Some of these tanks may cost diamonds, but if you want a tank that costs diamonds, just buy it with diamonds!
You don't need to complete any tasks or earn experience points in order to get different vehicles or guns. If you really want these vehicles and guns, then go ahead and purchase them! They won't make the game easier or harder either.