Let me tell you a thing about HEPA. It has become a marketer’s wet dream that lets them prey on people’s fears of pollution and dirty air. In a nutshell, HEPA filters work by providing a random path for particles (smoke, gasses) to impact into and get caught in the filter material as they flow through a volume of air. The effectiveness to do this is a combination of the density and impact surface area of the filter material. But people read HEPA, and they think it’s a magical purification. For a layman’s summary, read this: http://www.cnet.com/news/appliance-science-hepa-filters-and-the-physics-of-fresh-air/
Have no fears, I will absolutely be adding charcoal to my design, I just haven’t got there yet as it’s still a work in progress as I improve my Sketchup skills. But I think you have the purposes of HEPA and charcoal backwards. Charcoal is what removes the toxins, and it’s important to use activated charcoal rather than plain charcoal. HEPA removes the smoke.