113 hours of rendering time for such a big one doesn't surprise me very much. Regarding your dpi "problem". In fact it doesn't matter really what you select there. Important is the amount of pixels. dpi only tells you how many pixels you will have on one inch. Nothing more. You need to have these pixels which you can spread out. You must look at it from the other side. If you want to have a specific size of your image, with a specific amount of pixels per inch (100 dpi or 300 dpi for example) you can calculate how many pixels you need to render. It is obvious that the more pixels you have per inch, the better the quality of the printed image will be. But as i said, the amount of pixels is important. You can change the value for dpi in most of the paint programs (like Photoshop). So don't worry about that.

One day your ship comes in - but you're at the airport.