I know this is a loaded question, but how pay to win is this game? I didn't see anything directly mentioning this in the OP. Not that I mind dropping some money if it's good enough I just want to be prepared for the inevitable paywall if one comes up.
You can definitely buy your way into a powerful team. No way around that, the entire genre is 'pay to win' by its very nature. But the only reason to have a money-fueled team of super merged monster units is to permanently stay in Tier 20 of arena, and the rewards for doing so are almost laughably insignificant compared to the cost.
Also, I'm about to make it into the highest tier of arena running a team entirely composed of free heroes. Sure, it took months to make, and 3 of the units are grand hero battle rewards (you'd need to wait for them to run those events again), but it's doable. And they are hands down my strongest characters, I clear most of the hardest PVE content with ease.