Touchscreen-- yes the touch screen that in your second post will work, the one in your first post was only the touch interface. I was unsure on how much fabrication you wanted to do, so i was recomending a simple(plug-and-play) touch screen monitor solution, in case you decided against cutting up your dash bezel.
the other day, I was looking at a build log for a mini cooper that the member had the same idea(bad laptop screen-great carputer). look around in the build log areas just to get some ideas on what other laptop users are using to mount theirs in their cars and mounting locations.
the ultimate choice for mounting location is yours. it definatly looks like it would fit under the seat, the good part is that it is a location that is heated and cooled, the bad part of that is it is susceptable to messy kids, and spilled drinks, and tollboth change, and you have to unbolt the seat to service it. the trunk is also great, as there is enough room, the good side is space, and it would be easier to sevice, the bad side is that the space is not heated or cooled, and all wires would need to be extended to work properly.
wheigh your options and go with the best install location that works for you. just remember that you are the one that will need to maintain and service it, so for me or anyone else to tell you what is the BEST mounting location, could end up costing you later (or just cursing the idea of ever listening to some moron on a online forum).
there is tons of info here, so look around, and enjoy building your project.