Dampness at bottom of exterior walls

[Kent Verge]

My current rehab has an addition with a concrete foundation at ground level. The exterior walls have moisture at the bottom that led to mildew on the bottom part of the sheet rock. I removed the bottom 2' of sheet rock throughout and removed the damp insulation. There were also some nasty cockroach nests inside along with some geckos (they eat the roaches).

I know that there is an issue with drainage/grading outside that I need to address, but since the addition is at ground level, should I also add Z-flashing under the siding? Is there anything else I need to do about the roaches (other than a chemical barrier), or will resolving the moisture issue keep them out?

The cottage in the back is also on a slab at ground level, so it probably has the same problems.

This room is also where the previous owners fed the feral cats -- we caught 29 of them. They had sprayed on the walls, so I hopefully removing the sheet rock will help with the urine smell too.