I'd start with the outside easy stuff first and work my way in.
- Wheels and their hexes
- Motor pinion gear set screw tight
- Pinion/spur gear mesh
- Universal Joint Cups (front/rear diffs) set screws tight
- Dogbones move freely when steering is at full left and right
If those all look good, then it's time to look at the diff gear internals, and inspect the teeth on all the gears.
As far as the aluminum arms go, I'd stick to the plastic ones. I would rather have a plastic arms snap, or break than have aluminum arms all bent up, and yanking on the chassis. It all depends on how hard you run your RCs as to how much rigidity you are willing to risk.