Thanks for the advice!
I'll start with 1 air filter. I suppose I only use 1 filter for my entire house, so 2 for a small room would be a bit overkill anyway.
As far as the flexwatt, it's what I use in my incubator currently and it seems to work really well actually. I'm not planning to heat the room with this "heat box", but simply to heat the air somewhat before I blow it into the room.
The boxes primary function is to blow fresh, filtered air into the room. I was thinking of doing it on a timer. 7 minutes on, 53 minutes off. The room is only about 600 cubic feet and the fan blows 90 cubic feet per minute. So, once an hour the room will get a full flush of clean air that is somewhat warmed up. I may scrap the warming feature of the box altogether and just use it as an air filter - we'll see.
Trial and error is a fun game
Once the box is built I'll be sure to post pics!!