I am ignoring the potential for patent infringement.
Let's take the example given and see what can be protected.
taking the chassis of a name-brand automobile off its manufacturer's assembly line, significantly modifying it (in appearance & function) to instead use it as the structural foundation/base frame for a large commercial air conditioning unit (of the size typically used on the roof of a large building) (emphasis added)
First, if you make a significant modification, that should be enough for patentability, especially as we may expect that this modification is totally unexpected for the original use. An example of this is if you add stiffness to reduce vibration, but in a way which would damage the handling of a car using that modified chassis.
Second, it sounds like an air conditioning unit using that chassis is a novel and inventive product. An example of this is an a/c unit with wheel wells through which the piping passes, instead of their usual layout.
So at least there are two options for patentability:
New component has a new feature not found or obvious in view of art
New system which uses this rather than another component.
One can also envision that a method of using the component would be patentable.
In my experience, when you take a component and apply it to a new use, engineering considerations usually require significant changes, which are often patentable.