Or is it better to hold of a little with Octane 3 around the corner, or can I upgrade for free?
You will be able to use the test releases of Octane 3 with your Octane 2 license. However I am unsure what the upgrade policy will be once Octane 3 comes out of beta - there might be an upgrade fee.
Does the plugin use the shader tree like Modo, or does it work like the Octane stand alone? Untill now I mostly used Modo to model, but I hoping to change that
I find the shader tree in Modo a bit weird.
The plugin reads the materials from the Modo Shader Tree, but you can override any material and set it up as you would in Octane Standalone (with nodes) - and the plugin will convert the Modo material for you in this case as best it can. See the video at
http://render.otoy.com/forum/viewtopic.php?f=34&t=42581 for details (although the plugin has progressed a lot since that video was made).
Does the animation work? Nothing fancy, rotate, translate, camera focus, maybe some materials?
Yes, everything can be animated, and camera and object motion blur is supported in the plugin. You can also export animations from the plugin to OCS/ORBX to render in Octane Standalone if you need vertex motion blur, although I'm hoping the Modo901 API will support object motion blur in the plugin very soon.
You can also try the demo version of the plugin. I have a Modo901 version of the demo too if you need (PM me for this).
Paul