I've flipped multiple properties with mold. My current flip was a mold house, too.
The first thing you need to do is eliminate the water source. Whether it be a failed sump pump, broken pipe, or leaky foundation. It needs to be solved FIRST.
Then, I fully gut the property, including insulation. You need to get rid of the drywall since it's porous and holds in a lot of moisture. If the insulation is wet, that needs to go to. Let the structure of the home dry out. A majority of the mold will likely be on the drywall, but you will still see some mold on the inside of the exterior sheathing, floor joists, 2x4s, sub floor, etc. For that, it has to be chemically treated to kill the mold. I use this product: http://www.traskresearch.com/mold.html. Pricey, but kills the mold. It's applied with the 2600 fogger on this page: http://www.traskresearch.com/order.html.
Once I've applied the product THREE times, I let the house completely dry down with industrial fans, then do an air quality sample with a third party (not Trask Research since I want unbiased results). If there are not excessive levels of mold in the property (using the outside local atmosphere as a benchmark) I begin putting the house back together. If the levels are still excessive, more remediation, more chemicals, more fogging.
Bleach does NOT kill mold. It removes discoloration - and nothing more. The spores are still alive and active with bleaching method. Kilz primer does not kill mold either. If there is exposed wood which will not get recovered with drywall, but is stained, I use Zep Mold and Mildew cleaner to remove the staining, after the area has been treated with the 501 concentrate.
Mold remediation is very labor intensive since it typically involves fully gutting a house. If the mold is as bad as you describe, plan on removing all drywall, all cabinetry, all trim, flooring, etc.
In my current project, I even changed the sub floor since I don't want any trace of mold in the property. In total, I've filled up four 30-yard roll-off dumpsters with debris from the 2,600 square foot home.
And, of course, when you're done, DISCLOSE, DISCLOSE, DISCLOSE.
