Free to play, you definitely need to have a good plan of action. People who spend will have an advantage over you, but you can still succeed in arena due to all of the different payout hours and the AI being horrible on defense. I have both an account I spend money on, and an account that's FTP. I'm able to finish at the top of arena in both.
This game also adds a lot more content as you progress in level. Between raids, terriroty battles, and other events, there's always something to take up time.
At 54 you just unlocked mods, this is what separates good teams from bad teams. You want as much speed as possible on your arena squad. Start farming health mods, and look for speed secondaries. Additionally equip everyone with a speed 'arrow' primary. Research mods before the rest of your shard and you'll shoot to the top if you also have good characters with maxed gear.
After you learn the game you could also choose to restart with a better plan if you so choose.