Well, chlorella gives you ego, which is your hp.
As you upgrade your girls affection, time intervals are raising, so you will have to collect money like once a hour or rarer.
Yeah, this is confusing, but still you aren't forced to spend cash on it, it doesn't take long to collect money on your own.
Girls really is a challenge to acquire, but it's just a matter of luck, eventually you will get them all. As for "free game currency" what do you mean? there's "money" that actually a free game currency, or did you mean "kobans"?
And about time sinking, you need to know that this game is not for active playing, instead it's good background game that doesn't require much time and attention. But still, it's up to you to decide to play or not.