I am writing a finite element code for my final year project of BS Mechanical Engineering. The geometry is an integration of several parts composed of different materials. I don't have exact values of material properties. However, I do have ranges of them. For example, for Steel1020, Young's modulus varies from 205-215 GPa. Should I use lower limit of Young's modulus or higher limit of Young's modulus?
I would suggest you choose an experimental design approach and use a half-fraction factorial design to cover the min-max intervals for each of your materials. You will get a set of solutions that you can then examine to determine the best and worst case situations.
In general, for geometries that are complex enough, it is not feasible to determine the optimal choice of material properties for a given functionality by inspection.