Selling an entire render farm and replacing it with a low-end $200 graphics card is a big ask. A Titan is somewhere around 7 times quicker than a GTX 650 Ti - so if you are rendering big animations, you need to look at getting more firepower.
From a plugin settings point of view, it's a little hard to make suggestions without seeing a sample frame which is grainy. But in summary.....
1) Set caustic_blur to 1.
2) Set hotpixel_removal to 0
3) "IBL sunlight" is not possible in Octane - it is either IBL /or/ daylight lighting. Daylight seems very very fast. Not as fast, but will depend on the image map you are using.
4) Using a single plane for a background goes against the "Octane way" a bit. IMO you are better off using the IBL as the background (adjust the Poser camera focal length to get the size right). If you have a background plane, it will be cutting off the light from the IBL image, so the final render quality will be compromised.
5) See if "direct lighting" gives you a high enough quality render for your needs - it is many times faster than PMC/PT
6) You can increase the amount of scene lighting (by increasing the daylight or IBL power), which will remove the graininess quicker, and then "imager"->"exposure" and "gamma" to bring the image back to the light levels you need.
7) Under "kernel" you can reduce the "maxdepth" quite a lot, and then adjust the "gamma" to compensate. A "maxdepth" of 6 will render faster than the default 16.
8) Are you using an SSS skin shader? If see, probably not needed for animation - use the standard skin shader the plugin converts skin materials to.
9) If you have any light going through (specular) glass, that will slow the render down, and there are tricks you can use to speed up that situation.
Hope that helps.