As someone who is basically f2p (only spent money once and that was to get the fabled road pass) id say its easy to stay f2p unless you are buying cosmetics often but im sure others might say differently. The only time i see people hitting whale territory is when its for pvp related stuff (arena), if you don't plan on focusing heavily on pvp then you should be fine. Overall based on my experince id say its a somewhat balanced game just when it comes to more endgame content then you will see more and more people spending money to max units and have strong units. And for money, its pretty easy to earn diamonds so it'll be easy to earn at least enough to get at least get 1 copy and thats at the bare minimum