We usually always used poplar for paint grade casing. It doesn't show grain lines under paint (like Doug Fir does) it's fairly stable (once painted) and is semi hard, almost like maple.

It has an unpleasant yellowish green color so has to be painted, no stain grade -- it's one of the less expensive hardwoods.

Clear pine is also good, doesn't show grain lines under paint, but clear pine can be spendy, almost as much as poplar, IIRC.

Any 1x4 will work pretty much, doesn't have to be specified as casing, though something with sharp corners would be good, IOW, no 1/8" quarter rounding on edges.

Reason for this, on the butt joint above, the top cut edge of the vertical will have a sharp front edge whereas the quarter rounded edge of the horizontal piece will sit back slightly from that edge. One way around that would be to use stock that's 1/8" or maybe 3/16ths thicker for the top piece. I've seen it done this way a few times. Makes life a little more complicated on your stock and tooling, that is, a planer comes in handy for that if all of your stock is 7/8 or 1", you plane it down thinner for the side pieces.

If you have un-rounded material, just touch the edges with sand-paper very lightly before assembling to get any loose burr off, assemble it, and then use a sanding block to slightly round the edges that are exposed. That will give you a nice smooth joint at the top butt joint.

One way around that, if you have stock with way rounded edges, is to use the rosette corner pieces also on that page you linked to. Those are pretty easy to use -- easier to get good results with those than with mitering. Has a sorta old school look -- I've used them on a couple of remodels to match the rest of the house. You cut each piece of casing about an eighth shorter than if you wanted the two corners of the casing to meet (as in their picture, I don't like it that way), twice that for the top piece, it meets two rosettes, after all. That allows the corner of the rosette block to protrude in slightly from the two casing pieces.

Home depot usually has rosettes available.

For the apron, I have a thing for making it slightly wider than the casing, about 20 % wider. This from having seen a house once with wide casing and a narrower apron. Looks really weak that way, sorta like a weak chin.

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