Maybe a good, nonintrusive solution would be to do what the cookbooks do and print a table of material substitutions and a table of size equivalents in the front. Neither would have to be very large. You might even throw in a few articles about modeling with styrene or wood, from later magazines.
Yes, a conversion table might prove useful to a certain degree. However, older articles often specify certain manufacturer's castings, details and parts, the equivalents of which may not even exist today and suggested substitutes would be necessary to make the project(s) viable today. It's an all too common situation in regard to articles decades old.
I see the potential for similar problems by simply re-printing E.L.Moore's articles (or any collection of early projects) verbatim in this day and age when the scatchbuilding skills of the average newer hobbyist are dramatically less than in Moore's time. Serving simply as a source of inspiration won't cut it in much of today's hobby, where its newer, younger, element is mainly familiar with simply gluing several plastic walls and a roof together. To sell well in such an enviroment the printed material must indeed lead the reader by the hand through the many necessary steps.
I've run across the part-unavailability you mentioned. It can cause some trouble with structures, although it's not so bad as with rolling stock.
Moore's articles tend to be far less troubled by it than average, just because he kept the use of commercial parts to a minimum, no doubt to keep costs down, and used simple stock materials that are still available and easily substituted. You can still get sheetwood, and cardstock, and clear plastic, and Northeastern siding, construction paper, and dry ball-point pens, and even the Northeastern corrugated siding he used as a forming die for paper (I always wonder what NE thought of that one.)
Windows and brick are the only exceptions I can think of.
Moore inked windows with a ruling pen. Drafting suppliers can be hard to find, but the pens can be bought online. Windows can be inked quite well with a Sharpie, or run off on clear overhead-transparency stock from Scalescenes or home-drawn images; commercial castings can just be dropped in, too. (Moore actually used ready-made windows on at least one structure. It added a few dollars in cost and he stated that he was "unlikely to repeat the experiment". )
Moore used Northeastern brick, or Walthers brick paper. Unobtainable, but there are plenty of substitutes. I use Scale Scenes brickpaper.
Now,as for the necessary skill, I feel that it can be developed pretty easily, once you get started. We were all newbies once; of course, you've gone waaay farther from it than I have, but maybe I remember the first steps all the more clearly for it. I was reading Raymond F. Yates' How To Improve Your Model Railroad, chapter "Models for Almost Nothing!" I think I must have been about 11, and as it turned out, I had almost exactly that much pocket change. There was one image, duplicated from "The" Model Railroader, which showed the basics of simple cardboard buildings - cut out the walls in a strip, scribe siding, score corners, glue together, cut windows and door panels from more cardboard and laminate behind walls. There was a little bit about measuring prototype structures and scaling them down. That was all it took, so I grabbed a cereal box and started cutting.
What emerged was an odd freight station, a basic gable box with a door and some windows. Siding was clapboard...I figured "Hey, real clapboards are like this" and cut out a bunch of overscale ones, overlapping them until the whole thing was covered. I think they were cut from posterboard. The whole thing was plopped on a piece of 1/4" thick wood as a platform (it was an odd bit, laying around the house) and painted -yellow, I think- with poster paint. Odd bits of wood made piles of crates, and it was done. "Hey," I thought, "I kinda liked that, but it looks a little big." So I went back, learned a little more about scale from some article or other, and built the next one better. I think the yellow house here was my third structure; I wish I'd saved the first two.:
I suppose that's why I don't favor a lot of hand-holding. I don't often follow preprogrammed steps. I suspect a lot of the people who want to build things are that way - a lot of the value in scratchbuilding is the creative freedom it gives, and people who value creative freedom highly often aren't good direction-followers. Learning to scratchbuild, for me, has been an iterative process of finding inspiration, doing some work, finding solutions to problems encountered, applying the solutions, and so forth.
I've always learned best that way, whether it's been mathematics, computer programming, CAD, or cooking. Instruction inspires action, and action in turn helps me understand the instruction. This kind of learning tends to stick.