Eeeeeehhhhhhhh...
one by one:
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I would like to use Horde as a renderer of static images.
That's doable. You could use more complicated shaders and pipelines.
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I would need to render the scene using soft shadows and full reflections, using if possible hardware acceleration.
You'd have to write shaders for soft shadows, you'd have to write shaders and C code for reflections. Of course it'd use hardware acceleration.
Commandline, no brainer.
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My question is, is there a format understood by Horde that can describe an entire scene?
Yeap, the scene.xml.
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once i've got my scene ready, how can I render it on an image file using Horde?
Render it to a buffer, grab the buffer, write to file.
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I would prefer not to have any windows open on the screen.
Eeeehhh...not even for a preview? Sure you can do this, just never swap the buffers.
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I'm new to 3d and I guess this could be a nice start.
Horde isn't an offline renderer, so you can't expect to get super high-quality results without putting in a lot of work into pixel shaders and pipelines. You'll have to do your own global illumination, ambient occlusion, your own everything, in shaders and pipes.
But you can do it. I don't think it'd be a good project for someone starting in 3d though, but that's for you to decide.
EDIT: I just dug up GPU Gems 2 article "Implementing the Mental Images Phenomena Renderer on the GPU" they mention specifically that shader compilation of many heavy duty shaders can be of such significant overhead that software rendering is ideal, but that if shaders are already compiled, and textures fit in memory, it's fast as ... fast.