coming together but main thing is unless you have another filter running on this DO NOT put that pump on a timer! if it is shut off there will be no conversion of waste. filters need a continuous flow to work properly.
couple other things i can think of. first foam i would add some floss or something finer under it so you have a 2 stage mech. 2nd foam, quick question, how you getting that in there

.
i don't really know anything about using c02 reactors so i'll skip that for someone who does.
for the bio i would use scrubbies just b/c their awesome(read some arguments for bioballs but i think its crap

) massive surface area and cheap. space under doesn't really need anything or may reduce flow over time, but i have seen sponges placed right before the pump area.
lastly heaters. i think there's probably different views as to what is best but IMO i would place them both under the bio(assuming you meant 2)-my reasoning is there the water slows down so more contact time and the water will not lose the heat you just added as it flows over the bio.
hope some of this helps and look forward to seeing this running, i think except for the lost space, in tank sumps are one of the best ideas(best being the complex sumps hidden in basements/different rooms/ under benches

etc) out there. no chance of flooding, look cool, and should be easy to clean and tinker with