3D Graphic Design

Industry: Technology
Design details: Good morning,

I currently have two sets of the following (one for iPhone 4s and one for iPhone 5):

.OBJ file
.MTL file
Texture file in JPG format

The model contained in the .OBJ file comprises two sets of polys representing the phone itself and a case. The phone is, for want of a better expression, already housed in the case.

What I am currently doing with the model is loading into an app and then amending the Texture file to include a custom image for the case. The problem is that the faces extend around the sides where I want to only apply the image to the back of the case.

Therefore, I require the following:

The .OBJ file amending so that the back portion of the case visible to the user to be represented as it's own face.
An amended .MTL file
An amended texture file

Finally, the quality of the model is quite low. I am using OpenGL to render and rotate the object. Even when operating at 60fps the lines of the model are quite jagged and unsightly. Is there anything that could be done to improve this quality.

I will supply the files I have described above.

Could you please provide a cost and timeframe? Similarly, could you please confirm what the cost would be to replicate the above for different phone models i.e. a iPhone 5c/Samsung S4 etc etc.
Ideas for the visual style: See description
Things to avoid: Creating a new model from scratch
Extra notes: Preference will be given to those candidates who can deliver quickly.