What if our whole body is made up of cancer cells?
No.

One aspect of cancer cells is that they no longer follow their programming, ie they stop doing all the work that a normal cell in that particular part of your body would do. 

The next aspect is that cancer cells keep growing even when they are told to stop, even when there is no more space to grow.

Cancer cells also tend to destroy surrounding extracellular matrix in the quest for more nutrient. SO stuff like bone get deformed or just broken down leaving holes. 

So if your entire body were made of cancer cells... well it would fall apart as cell stop doing their assigned job in the body, start growing without care and break down existing structure for better access to more nutrients. 

What you want are immortal cells that still do their job.. stem cells.