The DM600 would be just fine for your dump truck. That's what they are designed for. Yes, the entire engine & transmission should be gold. I found that Tamiya Gold Leaf X12 works quite well. Here is my ENDT 675 Maxidine from my Mack R685ST:
For the interior green, I've heard that Humbrol 120 Matt Light Green is a good match; but I've never used it.