Previously undetected issues on cash only deal

Hi ll,

I am closing on a cash only deal in a couple of weeks in a very hot market in Philadelphia. The property closed showings after 72 hours, and called for best and final within the next 24 hours. Knowing this I brought a contractor with me. We walked the place and he said it would cost $40k-$50k to repair. I used $55k as budget for offer. I did another walk thru yesterday, broke down the numbers and now we are looking at $75k. I'm not a happy camper. Anyway, there are a couple of issues that concern me. Dampness in basement, step like crack in stucco in back, 2 small leaks in roof - I also want to know whether a wall is load bearing. Should I call a home inspector, structural engineer, or should a better contractor be able to tell me?