You might be best off with an Epia board. They're nice and small (17x17cm), low power, but still reasonably fast for what you might need.
For display, if you are going to be having any kind of text, then you will want a VGA display. Such displays would be from DSCustoms, DigitalWW, Xenarc etc. The cheap LCD's you find on Ebay and the like (such as for $100) will not be VGA, and will only have composite in, which will be very bad for any type of text, and are normally STN (which means low viewing angle, dim, washed out colours).
Having touchscreen might solve any input issues you might have, however if you will not need any input, then just get one without touchscreen, which will save you a few $$.
For power, you should be able to find a dc-dc psu which will take 24Vin, otherwise, you may wish to use a voltage regulator, and a dc-dc psu that takes 12Vin. The Epia motherboard is ATX, so what you say you've found should be OK.
As you will be offroad, I seriously recommend you look for a laptop (2.5") hard drive, as they will be much more shock resiliant than the standard desktop (3.5") hard drives, and will also consume less power, and put out less heat.
As for casing, I would not recommend a totally sealed enclosure, as even though things will be low power/heat, they will still put out some heat, and if there's no way for the heat to escape, then you will fry your components reasonably quickly. Even just a small 40mm fan on both ends of the case to move some air through (doesn't have to be that much air, no need for a tornado!) will allow the heat to be dispersed
Good luck with the project, and if you need any more specific help, chances are someone has done something similar or identical with similar/same components, the search button up the top will be your friend
Welcome to the forum, also
Garry