As far as I know Max9 is the only program you can use for animated meshes (with Runic exporter) so you're in luck.
You need a root bone, with NO verts attached. It needs to be at 0,0,0 so your model will appear in game where it's center is.
Ogre (the renderer) only allows 4 weights per vert (ie: the vert can only belong to a max of 4 bones - not a big deal really, it would need to be a pretty complex rig to use more).
It needs to be a skin modifier, biped wont work.
That poly count is probably fine, I imagine you wouldn't encounter too many dragons at once. But as far as that goes, Torchlight is low poly so it can work on a wide range of machines. Ogre can accept a very large amount of polys, you could get him in game just fine with 60,000 or more. But that would kill performance on some machines.
There is also a released pack of alot of Torchlight stuff in obj and Max9 format so you can look into it more, but all you really need to do is:
rig, animate, export each anim seperate (but they can all be keyframed together in one file) and export the mesh then convert the xml to mesh (ogre command lines)
have texture in png, tga, jpg or dds format and a material file (shader file) for it. Runic didn't use any special shaders, just diffuse and maybe spec or alpha on some stuff. But with Ogre shaders you can add normals, scrolling, rotating, etc...
Then you'll need to look into the pak file in Torchlight and extract some files of a character to see how the dat files are set up, they call anims, etc... you can edit in notepad++ or in torched, I haven't had luck finding model directoris in torched yet, then probably open or create a mdungeon to place the character in. There's also a way to add it to a list so they will just spawnalong with other creatues I beleive.
Good luck, he's a pretty cool dragon.