Sounds like a daunting deal for a newbie - especially if you don't have construction or architectural background. You've done the right thing in bringing in a structural engineer and contractor to look at. I did not read or see that you've brought in an architect - sorry if I've missed it. It would be a good idea to have an architect review the mold condition and see to what extent the damage is and where the moisture penetration is coming from. Hiring an architect, structural engineer, and contractor can run up your tab quickly. If you can find them to do an initial consultation for free that would be a bonus - or find friends who knows one. Get their recommendations, do the math and see where the numbers are in relation to ARV. This might be out of your budget (or anyone's budget) and you may have to prehab (like one person said earlier) and get out earlier and not lose money. Numbers are key here to be on top of this deal. Get quotes on mold mitigation (removal and cleanup), rehab numbers and then see if you can stay on top of profit margin. Good luck and wish you the best!