Best way to create/design modular Player

So I had an idea for the game that I am working on to have a the main character be completely modular and upgradable. The player is a robot and I would like to have each piece (torso, head, legs, left arm, right arm) to be able to be switched out and upgraded with better parts.

I am wondering what is the best method to go to create the different parts and have them be assembled at runtime. I have started with just basic unity native shapes as prototypes for the different pieces.

What would be the best way to accomplish this without killing the draw-calls on the iphone/ipad/android?

One of the best ways is to model your character with ABSOLUTELY EVERYTHING ON HIM! Then, just make everything not relevent invisible. This is a really good, quick method, and allows you to have lots of items with little speed impact. (Also, the character with all the suits etc on looks awesome!)

you can create bones in the position that you need your Armor / Weapon to be instantiated and use them as Transforms.
never tried it but i imagine that it works since these bones will move and rotate with your character