Here's where I started from.
As you can see, I armoured up the cab of the excavator, and wrapped some heavy chains around the tracks. I also added a piston to the excavator arm just to bulk it out a little.
I actually used the base colours of the original vehicles, but applies washes and highlights to add a little depth and, to some extent, weathering.
As you can see, the excavator rotates, although I have glued the arm into a fixed position as otherwise it flopped around in a rather unaggressive manner.
The total cost of this beauty was $5 - $3 for the truck and $2 for the excavator.
Here it is in action, chasing one of last week's purchases (now called 'Dark Hydraulic') across the post-apocalypse landscape.