Tooling costs for any RTR locomotive are massive and for the most part they cannot be split without destroying the original tool. These are engineering-standard components that cannot be changed without destroying the ability of the tool to be a tool.
The only exceptions to this are some tools that are designed with slide-in sections to represent detail differences (headcode boxes and the like), I believe that Dapol is the most common user of this practice. Still, such an idea must be taken forward in the design stage, and not retrospectively.
So you couldn't use the front of an A4, the boiler of an A3 etc because you would eiher have to cut up a tool (in itself, not the easiest of tasks) or create a brand new one from scratch. In either case, there isnt a whole lot of cost saving there.
The same problem with the B17, there would have to be an entirely new tooling for the body.
Fraid that this idea is a bit of a non-starter: its wonderful in principle, but I doubt that it will ever be able to be used in the real world