Best 3D modeling software for an easy construction of a vehicle body kit?

There isn't anything easy about 3D modeling at a beginner level. It takes time and the will to learn. In most programs you will have to create the bumper (and everything else) from a standard primitive, manipulating certain points and faces to get the desired shape. This is not easy if you don't know how to use the program. In most cases i recommend 3DS Max for this kind of stuff, but for you, i would say Blender is right up your alley. It's free and there are tons of tutorials for you to watch in order to get the hang of the program. In order to learn to make what your vision is, you have to learn the programs basics first.

Blender: https://www.blender.org/

Beginner Tutorials: https://www.youtube.com/playlist?list=PL9057AB8E3C...