Games Workshop produces two kinds of kits - metal and plastic. I hate metal kits - it is hard to convert them, hard to keep them unbroken, hard to repair them if they're broken. And Chaos Dreadnought is metal model.
Modeling Plastic Chaos Dreadnought!
All you need - Plastic Space Marines Dreadnought, few Terminator torso bits and few decorative chaos bits. And may be some greenstuff.