What's the best way to estimate repair costs?

[Monica Johnson]

I am looking at a house that's selling for 30k in the Atlanta area, comps are 150k.  I have a hard money loan for 60k.  I'm going to look at the house in the next couple days (the owner said he has to have it sold by April 3rd).  It appears to have some roof damage that has affected the drywall in at least 2 rooms in the house.  I wouldn't know where to start with estimates for that much damage and I don't know anybody who can eyeball it.  Should I find a contractor and pay them to come out and assess?