...Then if you get the BE III it would be good to switch over to the Monoplane model (after having to go back to the Biplane first since one cannot go directly from the Triplane to Monoplane)?
Yes, all of this is correct.
Also, one other quick question. Is there a significance to the bolder yellow border around the Better Engines III box (also I notice some bolder boxes in the Campaign game as well).
Yes, those are "breakthrough" technologies that require TWO cards to achieve (instead of one as all the others require).