I'm sure the plastic cutter works, but you can save yourself a few bucks by using a razor saw to make your cuts, and if you need to cut curves, scribing with the back side of your X-acto blade will do the job.
As far as the weathering products... they would work on a model car just the same as they would work on a model train! After all, the product doesn't know what it's being applied to! If it works on a model train, it'll work on a model car.
The "Age it Easy" stuff is a fine product, I'm sure... but I bet you could get the same effect for a lot less $$$ by simply using a black wash (thinned flat black paint).