First Deal Dilemma - now I get why everyone says to find a mentor

[Berish Edelman]

We are under contract on a SFH that has mold in the basement but is otherwise very well cared for. The house is being sold as is, listed at $100k, ARV is about 110-115k, offer of 82.5k was accepted. Estimating $100k all in after rehab, which includes adding a fourth bedroom, which should push the ARV to $130-140k.

Here's the question. We went over there today to complete the inspection - they had just gotten the utilities turned on - and saw water in the basement. The existing mold problem is almost certainly caused by the downspouts dumping water at the foundation, something we will address immediately after closing, but we saw a puddle of water in the middle of the floor, plus water around the perimeter of the basement. 

Turns out the water had been turned on at 10am today, and from that time until 4pm when the seller walked in and noticed, the toilet upstairs had been gently burbling over the top of the tank, running down to the floor, and finding its way into the basement. Stuck valve or something. This explains the puddle in the middle of the floor, but not the water around the perimeter.

We asked if we could leave a dehumidifier running - on their dime - so as to mitigate further mold growth until we close in a week. There is some hesitancy. We are toying with the idea of asking for a couple grand off the purchase amount to help mediate the water problems.

Would love to hear from the wiser and more experienced. 

Thank you!