They are to be built by Motive Power Solutions in South Carolina, and yes custom built. From that company's web site it appears they specialize in custom built locomotives and specialized rolling stock for railroads around the world. There is no mention on the company's web site of the new builds for D&S (yet anyway).
From what I understand the intended use of the new diesels is different then the existing diesels. They will be used as "road power" for hauling trains all the way up to Silverton. That will be quite a change.