I don't mean to sound like a dick, but I asked about my car and you offered advice about my boat, my house, and my girlfriends attitude.
It's really hard for me to do form, color, and shade one piece and then do another piece and make them match.
To complete your analogy, you asked about your life and then focused on your car. I pointed out that you had more than the car to worry about and gave you pointers one what to do about the entire set of problems.
This all comes from practice and is why I mention that you should work on one small piece until you have a firmer grasp of form, color, and shading and then extend your piece or work on a large piece from there.
Ideally you wouldn't begin with such a large composition and you would focus on the fundamentals that are required to accomplish your goal rather than trying to get it to look right first and then backwards working your way towards securing those fundamentals.
I started out this way. Took a lot of work and rework to learn that I should discard some of my ideas about how to go about creating a piece and adopt new ones even if they seemed really difficult. Most of the things one tries in the beginning are actually bad for your art and the process of making it. In the end it's all about practice practice practice though so keep going.
I really recommend that you work on a small portion of it and then move forward.
For knee and hip joints in robots look online at big robots. Zords from power rangers, Transformers, and Gundams all have this sort of blocky shape to them. They would be good examples of things to try.