A better idea is to find a good hobby store and check out Plastruct and Evergreen strcutral materials, although you can do some things with parts trees.
About the only plastic kits available are the AMT/Ertl/Matchbox Cat D8H dozers. I think Ertl has re-released a John Deere tractor backhoe,and the IH Payhauler 350 off road dump truck, there are re-releases of a Peterbilt/Challenge mixer truck, and several Mack and IH truck tractor/trailer combinations. All in 1/25th scale.
This might help...
http://scalemodel.net/
http://cs.scaleautomag.com/scacs/forums/
You'll want the Commercial section, although there is fantastic stuff in all of it.
Do a search in Google for scale models, and be prepared to be there awhile. Use the image search section too.
Also Google Modelleshow Europe and prepare to have your mind blown!
Those guys are
serious modelers....
Also see this site, DHS Diecast...
http://forums.dhsdiecast.com/
I've got more, this ought to get you started...
By the way, here are the search results for the AMT D8H, you can see how people customized the original kit.
http://cs.scaleautomag.com/scacs/se...=caterpillar+D8H+AND+sectionid:33&o=Relevance