I'm a university student and have to build a UAV that is capable of carrying a 100g payload and must have a total mass of under 7kg. It will be powered with an electrical motor, a speed controller and a lithium battery of maximum 2200mAh capacity.

The model will have machined / ordered parts and I was looking for more information on what materials would be suitable for the fuselage to make it lightweight but sturdy.

Thanks for any info

Cheap foam core board from the dollar store can do this size well. Fold, bend and use hot glue to assemble. Check out https://www.flitetest.com/ for a huge number of ideas, kits (if you want them), plans, how-tos, etc. Almost any design to fit your mission parameters available, and skill level in learning to fly it!
Easy material to make prototypes, very quick to build with, and super cheap to make mistakes with or replace a model as needed. Use clear packing tape from the Dollar Store to cover/protect it, and add a bit of strength (get regular, NOT heavy duty or extra strong). Get the Dollar Store foam board - better than from art/craft places which are heavier, costs more, and doesn't add any noticeable strength.

Are you sure about your numbers? That's a really heavy plane for a battery that size, and a tiny load for a plane that size. Usually these heavy lift contests have you carrying more weight than the plane weighs.