You don't have to spend real money for champs. You can unlock them by just playing.
Of course, that takes a lot of games, depending on which champion you want to unlock permanently.
You can still be competitive without spending a cent on the game. It might take a bit more time, but that's not necessarily a bad thing: if you play more to get to the same point as someone else, chances are your actual game experience will give you a little edge over them.
In the end, spending money on this game doesn't give you an advantage in power. You might be able to specialize on one champion more easily, and you might get some nifty skin for your favorite hero.
You might also get to have all those rune slots and masteries available sooner, but you'll still have to work for everything that will really make a real difference in power and skill.