The ESC and motor can actually handle a 3s Lipo! Only the fan on the esc has an issue with 3s lipo. The fan gets powered off the ESC. If you run 3s, it gets too much power and will burn out. You can wire the fan to be powered off the reciever, so it will not get more than 7v.
Battereis are rated by their nominal voltage. Fully charged is always higher. A 8.4v nimh can be like 9.6v when fully charged. Same as a 7.4v lipo will be 8.4....and a 11.1v lipo will be 12.4
Aluminum is a tricky thing. The suspension arms and shock towers are actually better plastic...as they allow for flex upon impact. Something has to give.
The steering hub and "C" hub...as well as the rear hubs are all good ones to upgrade to alloy.
Other than that, id leave the rest plastic.
The steering servo aint a bad part to upgrade. A metal geared high torque one (some on hobbypartz for $20) is good.
Also, it doesnt hurt to had a heat sink to the motor. They can lower temps by 20 degrees!
And as mentioned before, the ESC program card is helpful. With more powerful battery, upping the punch control setting and lowering the braking force will save the driveline and make the truck more drivable.
Whats the point of a 40mph truck, if it spins out every time you hit the gas or the brake?
You can also adjust the LVC setting. I think stock is 3.0v and I prefer 3.2v

Oh, another big one is the Tbone racing front bumper. Tbone racing makes bumpers for many redcats, and their stuff is on par with RPM! Their bumper is wider and has a better angle than the stock one. It will absorb alot of impacts.

Lastly is taping the inside of the body. Redcat bodies are PVC and not lexan. Its not as tough. So putting duct tape on the inside of the body (especially the front bumper and around the body hole mounts) will help keep it from cracking. Adding plastic washers (like the ones meant for the traxxas stampede) doesnt hurt either.

Hope that helps!