OK, custom enclosures are going to help alot. Instead of messing about with fans and non-condensing gases, you could fill the enclosure with oil. Just immerse the components. Then you have better cooling, no need for fans or vents (you could use a small pump but there would be convection currents caused by the hotspots like the CPU and GPU), plus the oil would also give some good shock absorption properties.
I would make the lower half of the enclosure a seamless unit with rounded corners. No sharp edges or welds to spring leaks. Like the original bottom case without all the holes in it. A rubber seal around the top would allow you to simply bolt the top half on. Easy access for maintenance, easy waterproofing.
Titanium sounds a good bet, with rubber around it to help prevent big dents and aid in waterproofing. Can you work your secret touchscreen solution into a setup like that?
The biggest problem with doing things this way (as long as you can sort out the screen) is going to be the battery.