Well, just think "How would I do this cheaply and get away with it" for a good enough "Engineering" approach for this case.

The really expert "Engineering" stuff related to things like maintenability, reliability, robustness and so on (which I myself am not qualified to talk about, as even though I have an EE degree, that's not actually the domain of Engineering I ended up working in so I haven't accumulated the professional experience that teaches one to take such higher level considerations into one's designs), isn't, IMHO, really necessary to understand to explain why those designing circuits commercially would chose the commonly available and cheaper components if they can.