Kevin, agree the labor cost will be higher in a custom shop building these but with care you can achieve good performance results far better than most mainstream mfgs. The trick is avoiding the typical North American construction of square edge single gasket construction and aluminum threshold. All the good sealing gaskets, thermally broken sills, multipoint locks, hinges etc come from Germany, Italy Austria. Insulated cores of all types are available there but costly to source here. Dan Palmer of H&H was at the Alpine workshops several years ago and since have developed some good door building systems.
Double and triple rebated doors are not that difficult to build even with simple tooling. In the Alpine Workshop we show how to make 78 and 92mm thick double gasketed door edges just using the Multiuse cutter in several settings on the shaper. Easy on a programmable shaper but also possible on a conventional shaper with skilled setups. I have dedicated heads for my own work but use this method sometimes for custom thickness.
the Fenesterbau and Bau shows in Germany are the best for high quality, thermaly efficient door and window building.
here is a Instagram link showing building a historic mahogany door then sample euro door sections at the end.
https://instagram.com/p/Ca493j3LOad/
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