Can one play the game successfully without spending real money for resources, diamonds, upgrades, etc?

In general yes you can. Almost everything that can be bought with money is simply to speed up your progress. So if you are happy to grow more slowly, work with others in an alliance to help each other, then you will still get to the same level as others who pay.

If you want to be the strongest player and play the game as a pvp game then you may need to pay money to keep up with others who do the same.