According to the calculation by volume on metrias excellent cake calculator - http://shinymetalobjects.net/cake/calculator/cake_calculator.cgi
- two 12" x !8" sheet cakes would give you 108 servings. I would suggest you cut them the long way, so that you have four 6" x 18" pieces, which you can then fill and stack for the body of the truck. I've made car wheels by covering donuts in black fondant and then placing a circle of in the middle painted with silver luster dust for the hubcap -it's all a matter of seeing whether you can find donuts the right size. Otherwise, you'd probably have to cut them out of a layer of cake with a circle cutter or a template and then cover them. As for sturdiness, I'd treat it like a two-tiered cake, with a cake base between the top two and the bottom two layers. I'd put dowels or straws in the bottom cake for safety and I think I'd sink 2 or 3 sharpened dowels down through all the layers for added stability. No ideas on the cabin, though - hope somebody else can help you with that!