One thing to keep in mind: units in 3D modeling apps bear no relation to real world units, they only describe the relative sizes of objects to each other.
With ODE, things are different. In order to do physics calculations, it needs real world units. One ODE unit equals 1 meter, 1 kilogram or 1 second, the standard international units.
It might be a good idea to adjust imported object sizes to these standard units, so you won't have to scale everytime you send something to ODE (or receive something back from ODE).
Kerkythea is a physics based render engine, and just like ODE, it expects standard international units like meters, In that regard, combining ODE for physics with Kerkythea for rendering would make an even stronger case for real world standard international units for the modeling/posing part of the application!

OGRE comes with some nice examples of how to integrate ODE. Raycasting is also a very handy utility, it'll help determine what object(s) the mouse is hovering over - very useful for selection.
Collision detection might be a challenge, especially if you want accurate detection between complicated geometry. Again, SubD is a great approach to have simpler (and thus faster) collision detection calculations, while the precision of the calculations will usually be enough.
The standard approximation of geometries by primitives won't cut it when you need precise collision detection, but it is certainly good enough for realtime animations or games.

The pen is mightier than the sword. But if you literally want to have some impact, use a typewriter

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