One of the Ciaphas Cain novels has a somewhat similar theme to this, with regard to the state of ill repair, lack of supplies and general asymmetry of forces. The whole story centres around a band of people scavenging what they need, occasionally coming across PDF deposits of ammunition, etc., or Mechanicus shrines, and otherwise gathering whatever makeshift weaponry and transports they can obtain. One of the best in the series, I reckon, because of the fact that it centres around the lack of equipment and the desperation of trying to fight a massive opposing force.
Having read that novel, I reckon can probably help with the theme.
That sounds about right. I had a little bit of a scenario inspired by Kim Jong Un's parades and the first step involves grabbing what they need.