If you want to buy, fine - but I think it can also work out great not to.
On one level E&P is “Pay 2 Win” and yet, unless you happen to be in one of the very top alliances, it isn’t so much “Pay 4 Wins”. Matches are made for alliance wars and in regard to which titans to face with consideration of the alliances history of battling other alliances/titans to date. In general you will always beat the same number of titans. Game tactics remain the same.
Sure, if you don’t have some heroes you may face a bit more challenge in certain events. Just remember you can always pick your opponents for raids and cup drop whenever you like. I’m settling for platinum tier with my squad still fielding one 4* and that’s currently fine for me. I typically clear up in 6 clean hits and a couple or three goes in the tournament and race through the chests. It all works out.
If a fellow alliance member buys heroes, great. They aren’t my competition. I still typically get into the A tier for titan loot etc. because I’m a good player. If other alliance members buy stuff then that just means we’ll do better in shared contests like wars where we all win. Happy days.