All of it can be done in blender. You should be able to open the .blend files with blender and then edit them with blender. Be sure you still give proper credit to the original creator of the model even if you change the original model.

Im pretty sure for attaching the hairstyle to the main mesh, you just import the hair after loading the body .blend file. Put the hair on the head and join the meshes together. If you need there should be basic blender tutorials if you just do a quick search.