I find your postulate to be both interesting and confusing at the same time. You are looking for heirloom quality kits without any components of the kit being fabricated in Taiwan (I assume you would also include other parts of Oceania) or made of delrin, a true space-age polymer. You then go on to wonder if Mont Blanc has plastic parts.
Well, I can tell you for certain that numerous high end manufacturers use "plastic" components. I can also tell you that many of the high end manufacturers use parts made in either Taiwan or other parts of the Pacific Rim.
I would also offer that "heirloom quality" is not something that is imbued by the manufacturer, but is something inured by the owner. I have seen many MontBlancs, Pelikans, Namikis, and Sailors treated by their owners as disposable pens. They only attach monetary value and if the pen does not increase in value (or at least hold the value) then the pen becomes a "workday" pen. On the other hand, many simple Parkers from the 50's have been handed down a generation or two and are kept with pride and reverence...so much so that fountain pen repairmen, such as Richard Binder, make a very good living repairing and restoring these lesser pens.
If you want to make an heirloom quality pen, focus first on making a pen with character. You can move further in the arena of quality and distinction by learning to custom cut each of your fittings and by designing your own embellishments. This is not a simple nor an inexpensive proposition, but with enough effort, dedication and investment it can be accomplished
As for kit pens that can be sold with a straight face for $200-$300, I offer that there are pens being made by members of this organization which are already being sold for those amounts and more. As testimony to that, I can attest to one member who has sold pens to senior management of both Mont Blanc and Omas.