Hi Mike,
well, modelling such "small airplane" with accuracy is a hard task.
I have been using AVL:
http://web.mit.edu/drela/Public/web/avl/ in the past. Just figure out what is the NACA profile for your wings and tail wings, "cut" the control surfaces and put the fuselage as a block and start to compute your coefficients. The drag one will be not so much accurate and the trust coefficient depends on your propeller (and eventually on your motor if you cannot control/known precisely the rpm). And in the end, you have to tune your coefficients
with some identification models....
On the other hand for Hardware in the loop testing or for getting a general idea about how your algorithms will work with your plane, it is fine just with obtaining some coefficients from well known planes close to yours (I guess it is close to a Cessna??), or work directly with Flight Gear Simulator or X-Plane (there are nice guides in the PPZ wiki or look at the ArduPilot forums/wiki), they have great models and you do not have to spend so much time on modelling stuff.