DDC's difficulty curve is basically: stages 1 and 2 = easy; stages 3 - 6 = hard.
Of course, there's lots of variation within that, especially depending on the difficulty mode you're playing on.
As for lives, they aren't spat out at you at fixed points or scores like with IN and PCB. Instead, every 5 autocollect bonuses is a heart piece, as well as every 2.0 bonus you get is a heart piece. There's quite a few points in the stages where you can guarantee yourself a 2.0 every time but that mainly comes with practising the stages and learning what works/what doesn't.