Before I dive in deep, one last quick check to get any other feedback...I should have another 50k to invest if this goes thru....

Third, the seller may die or disappear, leaving you with a legal mess that will take time to clean up.See a great lawyer with experience with MLOs.

Its a 4/2 4/2 duplex, 2400 sq ft total.The problem it is very hard to know how deep the mold goes without ripping out the dryway isn't it?

I live in FL so I know about having to use A\C year round.As you know people mention a brand and price but not model which will ultimately determine the total cost.I was in the exact same situation and due to the rusted coil tubing, and overall condition of the Air Handler decided for a total A\C replacement.Payne is considered "Builders Grade" lowest grade and some A\C guys argue not to install them because it reflects on their reputation when you have problems with the system.Look for the highest seer rated, quietest, best warranty HVAC that is fairly priced.The installer is a HUGE part of the HVAC's life span, along with your regular changing filters, cleaning coils when needed, cleaning drain lines and vacuuming dust out of the Air Handler Cabinet.

I probably need someone to invest with me but I don't know where to look for the deep pockets.  

There is a local guy who handles lawn, snow, and unit clean up who has a check mailed to him once work is completed.  

Also get the yearly contract for the 2 cleanings. 

Offer a cold drink on the hot days, hot drink on the cold days, help keep the property clean, even help shovel snow from steps.

We closed on the property on May 26th.As you can tell from the pictures the yard was a completely overgrown and we filled up two 30 yard dumpsters between the clean out and construction debris.  

Rule #5:  Don't Over ImproveYou want the house to be safe, clean and functional.