And Septic is not a big deal as long as it is maintained (you/your tenants must never put any bleach or chemicals down the toilets, tub or sinks). 

Then spray straight bleach, it must be straight bleach. on all of the mold even spray the wood studs. then wait for it to dry.

There are home remedy solutions with bleach that can kill mold and plenty of bottle solutions at a local home depot and it looks like you'd have some minor repair work.  

Dump in some bleach let it sit for a day .

with a septic tank you need to budget in pumping cost and with it being a rental I suggest pumping every 3-4 years. you also need to limit the amount of bleach products you use because it kills the anaerobic bacteria that makes a septic system work properly.

Here it is get the hottest water you can get and add 2 large scoops of all fabric bleach and KOSHER SALF, SEA SALT, OR THE KIND OF SALT YOU PUT IN A WATER SOFTNER a couple of heaping scoops. 

I decided to scrub the walls with bleach to try and "clean" the room before hiring professionals and was sick for 24 hours from the chemicals leeching into my skin.

I took everything out, washed the mold spots with bleach water and sprayed with special mold killer and informed my contractor of everything and requested that a mold inspection be done to be sure that it was all taken care of.

I've ripped out 30% of the walls, have treated the home with bleach, borax, and an 03 machine, but I want to ensure the house is mold-free before I list on the market.

The sellers did not bring in a licensed remediator - they brought in a contractor, who treated the area (a wood area, I believe) with "germicide and a bleach solution".