Hiya, sorry this place is so dead these days. In answer to your question: it's as strong as you make it.
The styrene itself is pretty strong, at least in slowly-applied pressure (ie, rollovers) but it will crack if given a sharp impact (crashing into a rock head-on). It will not have the "bounce back" that Lexan has, but it has one advantage: it can be repaired much more easily than Lexan can.
In terms of designing for strength: use nice thick styrene (.060 or so) and a good weld-type adhesive (Bondene and Tenax are two well-known brand names). This type of adhesive literally makes one piece of plastic out of two, instead of holding two pieces together like cyanoacrylate "super glue". You can reinforce the corners and window pillars with a second layer of plastic on the inside and it will help immensely.
Expect to get some cracks, and dab some adhesive into them to repair them, and expect to lose mirrors and door handles and little details like that once in a while, but in general, don't be afraid to drive the truck hard. After all, if you made it, you can fix it, right?