I do all my stuff in a program called openSCAD. It's free. But it's sort of a programming language. You write a script that describes how to create your object. The nice thing is it's easy to tweak things. But it's horrible for organic shapes like hair pieces. You can see my self-designed hair pieces are ridiculously simple. My male Trek characters all have simple hair pieces designed by me. The ones that have any texture are upscaled Lego designs that I've found on the internet. Invincible or any female characters, for example. Here is the code for the sword CC is holding in some of the early pics.
module sword() {
//blade
difference() {
cube([1.4,3,25],center=true);
// cut to make it pointy
translate([0,-2.75,-13.5])
rotate([30,0,0])
cube([5,5,15],center=true);
// other cut to make it pointy
mirror([0,1,0])
translate([0,-2.75,-13.5])
rotate([30,0,0])
cube([5,5,15],center=true);
}
//hilt
translate([0,0,12.5])
cube([1.4,9,1.75],center=true);
//handle
translate([0,0,15])
cube([1.4,2.1,8.5],center=true);
}
sword();
I think you'll find Blender is the best free tool out there to sculpt organic shapes like hair pieces. You might look into some tutorials on Blender for 3d printing. There are some videos specific to that. I've still not gotten into this yet but plan to use Blender where my CAD program is not a good fit. I find it quite good at modifying existing stl files. For example, I sometimes upscale Lego hair pieces. I then use Blender to cut the minimate head shape out of the hair piece so it fits better.
Learn the boolean modifier. It's the one you'll need to merge objects or carve one from another.
To directly answer your question, I'd say Blender is probably the easiest free tool you'll find for sculpting organic shapes like hair pieces. There exist easier to use CAD tools appropriate for designing the body parts and other blocky objects.