It was sitting directly on the concrete foundation and the walls are already open to replace leaky facet pipes.

I also recommend spraying the space with Lysol.One thing people don't think about is that there is usually a 1 foot high space between floors for duct work, pipes, electrical etc and/or inside the walls that the rats love to claim as their personal space.  

Your options vary by state.Asbestos appears in some ceiling treatments, wrappings around pipes, insulation, flooring and house siding.

I would paint the gas pipe in the front white where the background is white to blend it in.

Patricia, thanks for your exhaustive explanation.So we have a choice to hire GC to pull permits for us or pull permit as homeowner/builder correct?

I would exhaust all your free options first before making that large of an investment.

We haven't dug too much into the weeds of this duplex, but we were told we need new pipes for our waterlines.

This means when you do want to get credit, you may need to temporarily unlock your credit.The above steps don’t provide guarantees and are not comprehensive or exhaustive, but collectively they can substantially lessen risk.

Roof leaks, leaking supply or waste pipes, tub surrounds, cabinet bases (between the interior bottom of cabinet and the floor), all can create these slow leaks that rot wood and can "feed" mold and attract other pests that could be causing your issues.You might also have a professional perform testing for mold.

This is why I advise everyone to put HML as a plan Z, the absolute LAST resort and to only consider once everything else has been exhausted AND with EXTENSIVE consideration to assure, without a doubt, it will be very short-term and is ONLY a temporary bridge-financing.