There's a a small part about that in the SDK, i 'd say 100.000 is a good limit.

I don't think the SDK suggests 100.000 polis.
IIRC the stock external models are somewhere at around 15.000 faces in the main model, though some third party planes use up to 60.000, what is pretty high but ok if you have a fairly fast rig.

First, consider the FS world and all that needs to be accommodated by the system, such as weather and clouds, terrain, scenery, AI aircraft/vehicles, other management systems.  The model is but a small part and should not take any more away from the other aspects of the sim than are required.

Then consider your aircraft, complexity and number of gauges, flight managment systems in the cockpit, number of engines, and how the aircraft will be used.  Is it to be an AI aircraft or flyable.

In my experience, the number of polys should vary by type of aircraft.  High wing multi-engine aircraft will require more polys than low wing, single engine aircraft.

Large 2-3 seat cockpits versus single seat cockpits make a huge difference.

With that said, a well crafted low wing single engine exterior model may take 15-20,000 polys.  The interior model will require around 5000 polys for a 4-seater.  This assumes you only model what can be seen from inside the VC. (Using the Spartan Executive as an example)

Conversely, a high wing, 4-engine prop exterior model may require 30-40,000 if modeled efficiently yet with some nice details.  The interior model of a sizeable VC, large panel, with passenger cabin can be done with 15,000 polys including seats all efficiently crafted.  Without the passenger cabin, deduct about 8000.  (Using a Dash 7 as an example)

I believe the Beechcraft D18S came in around 38,000 polys for both the exterior and interior models.

My belief is that you model as efficiently as possible using no more polys than are necessary to create the illusion of roundness and smoothness.  Delete all hidden and unnecessary polys.  The more efficient you do the model, the more gauges and system complexity and vcockpit sections you can tolerate while maintaining smoothness of the sim.

Remember, we should use up no more "headroom" than is necessary so that we leave room for other aspects of the sim as mentioned above.

Hope this helps a bit.