Your buggy has several important parts to its manufacture.  You should produce a written/design project that includes all sections of the criteria listed in the exam syllabus.

The first section asks that you examine existing products, materials, processes and scientific principles as a research exercise and then use that data to create a specification for the product you are going to make.  With careful reference to that specification you should produce a number of designs.  These designs should be analysed and working drawings used to show features of your ideas. Remember that all ('technical') drawings should be done to include BS drawing conventions. (Drawing Conventions)

Your research might also sensibly include an analysis of why remote controlled vehicles such as your buggy are used.  What sort of environments might they best be used in and what problems would this bring to the designer?  Check out these sites and any more you can find. How can you gain additional strength in the chassis of your design?  You have only a thin sheet of aluminium. How has this aluminium been coated to give pleasant looking colours.  If you had been given a thin sheet of steel, how could you have prepared and 'finished' that material to give an aesthetically pleasing and protective end result? 
Use the list below to address the issues involved in the task you have be given.

Good researching...