Do not apply “rustproofing” to the underside of your vehicle, it does more harm than good. Most rustproofing materials are oil based, a very viscous sometimes tar like substance or a heavy wax like substance. These cause two problems, they trap any dirt or moisture to the surface they are applied to. Second they plug up the drain holes that water drains from the inside of your vehicle.
Removing the dirt, i.e. washing the vehicle is the first line of defense as dirt hold moisture. Remove the dirt and the surface can dry out. Applying a coat of wax to a clean surface will help by creating a barrier between the dirt and the paint. This helps mainly by providing a wear layer so that when dirt is washed off, it scratches the wax and not the paint.
There are some really good waxes now that will last up to 6 months or more but the best ones are a little more difficult to apply. I really like the crosslinked polymer (aka synthetic) waxes for durability.
Don’t worry about the engine, it is mostly aluminum and aluminum develops its own protective layer. It may not be pretty but it is effective.
Edit: When you wash your vehicle, run your sprayer along the bottom of your rocker panels, pointed up. The rocker panels rust out from the inside and not the outside where rustproofing is applied. Mud from driving during rain storms plugs up the weep holes for the rocker panels and water gets trapped inside. Using a hose end sprayer or opting for the panel wash at the carwash will wash the mud out and the rockers will dry out.