Get it as soon as possible, but if you're going to solo the items, then you'll want to be level 25~ to do it comfortably, or get a high level to help you.
the crab apron and damselfly worms are easy to get since most parties will go out their way to nab you one for your item if you ask. the ghoul is slightly more difficult since you gotta camp near a pop and then convince your pt to kill it.( it's hard to kill if they're noobs and easy to moderately annoying if they're decent/good at their jobs by now.
ya 18 to get. Mostly you'll get them while Xping in dunes. Apron and Worm I see alot while XP but not so much the Skull. might get a buddy to help you with this.