some people blow up copies of the engineering drawings you can find on the manufacturer's website or in the back of sales brochures for the real machines, to get true scale size for external dimensions and geometry. Internal design will have to be DIY but again manufacturer websites often show how the machines are built for real.
example - check out pages 16, 17, 18 in this Volvo brochure:
http://www.volvoce.com/SiteCollectio...71_2008-03.pdf