Hi Pogo.
I've seen a similar build on YouTube but it was a bit mucky, yet it had some good ideas.
One problem i would like to see be overcome is airflow, and cooling. A laptop is small, slow and suited for hence the name being put on your lap.
Another thing to look for is space. I don't know about you but having space and cleanliness inside a pc is a MUST and i will always step away from a spec of dust. If it is one of these 'M' GPU's you could possibly use a riser? Cooling that may be odd though.
With all due respects, doing this will certainly be a CHALLENGE. Have fun!
-C (A.K.A Dawn)