Is this leak evidence? Should we walk away from this deal?

[Vivian D.]

We had a plumber came out check the drain of a rental we had under contract, already passed option period just waiting to close. He found one leak that could cost us $1800 to fix (after we fix the foundation first). One hard punch. We did not have that in our calculation. Since we do not want to loose the EM and all the money spent on the house so far (option fee $100, inspection $450, and plumbing check $225), we thought to ourselves oh well, let's just go ahead. But then what the plumber said about the shower turn us totally off.

According to him, all the mold on the surface came from the moisture inside the sheet rock. As the house was built in 1989 and the way the shower pan came so low, he suspect there was leak below as well. So he recommends to remove everything to put in concrete wall & shower liner make sure no leak after. Even though his estimate could be too high (4-5K for the whole job) and we have not shopped for better quote, we would want to check with BP experts if it is actually necessary to remove everything. Just by the outside look, do you think it is that bad? Can't we just do a really good job of cleaning & get rid of the mold if you tell me that's not leaking evidence? Cause if he is right, we just gonna walk away from the house since there is no point to get back to seller ask for discount due to this new findings. Or can we?

Thanks in advance for all inputs!!!