Can anyone help me out?

I'm basically trying to build a site based around a toy construction site which allows the user to drive their wee truck around an isometric environment.

I've FTP'd a basic mock up to illustrate my problem:

If I've got a building on one layer and the truck with the isometric calculations on another, either above or below it, how do I get the truck to appear in front of the building when its in front of it and behind it when (you get the idea).

Is there any way of stopping the truck when it collides with and object. I'd also like to keep the path that the truck travels along restricted to the road. Is this possible or should I go back to 2D?

Any advice would be great because I'd love to try and fit in some detail to the environment. (I've made models of a mobile library and a stolen Ford cortina). Web design doesn't get much more fun.