A lot of half slabs for that area, but cobble is easy to come by. What I forgot after watching this video again was all the slime for the pistons. Need. More. Slime. A lot of the cool stuff needs sticky pistons in Minecraft (i.e. piston doors).
I guess if you undertake this project with a friend, gathering slime wouldn't be too bad. Hope to see this underway soon. The lava was a great idea, Doom, btw.