A readily available supply of O scale decals....for most everything except letters/numbers in appropriate fonts/colors...is, indeed, most of the concern with this concept.
That said, I'd prefer products that are painted/unlettered, as opposed to unpainted.
Reason? It would help the modeler if the manufacturer had cleaned the plastic/diecasting parts as part of the finishing process, and put on a coat of paint that would serve as either the finish coat (steam...black) or as a primer coat (diesel/electric....light, neutral color). Some of the molding/handling materials that end up on the 'unpainted' products can be the make-or-break of a good finish if not thoroughly and properly removed. That's sometimes a hazardous process for the typical modeler, if not a nettlesome one.
Ergo, I'd vote painted (primed)/unlettered.
But, then, I have too many other projects weighing down my 'bucket list' to consider adding this one.
KD