Thanks for the very informative answer and suggestions. In response to one of your questions about why I think I need such a high end graphics card, I need to do "real time" (ideally at 30fps) processing of 3D data. My current prototype (which isn't ingress protected in any way) runs at around 10-15fps, and with the GTX970, performance bottlenecks are no longer on the image processing side of things (uploading the images to the GPU now takes more time than the calculations), which they were using an older GTX650ti. Unfortunately, moving the data elsewhere for processing isn't an option - there is no guarantee of decent connectivity and the amount of data will be at least 25GB a day. It doesn't need to have a permanent display though, which makes life somewhat easier!
I can certainly make the box larger as you, and other slashdotters, suggest. In fact I've not "been" painted into a corner, I've painted myself into a corner and am always very happy to look for alternatives as you suggest. I'm a big fan of the keep-it-simple mentality! Ideally, an off the shelf solution would be perfect, but having talked to a few of the embedded pc manufacturers as well as endless googling, it seems that I'm going to have to design/build the solution myself. I very much appreciate your answer and clearly you have a thorough understanding of the types of issues I'm facing, and in a way I'm glad that I haven't missed something blindingly obvious!
I'm also realistic in that I don't expect the first prototype to work without hiccups, but if I can incorporate as many of the suggestions as possible in the design, then I can hopefully keep the failure/damage/rebuild rate as low as possible.
Thanks again for your post.