I run numbers based on the listing and do a VERY quick down and dirty estimate of rehab based on pictures and listing description.  

Rough estimate would be $65 per 4x8 sheet of drywall. 

Scott The Book on Flipping Houses, The Book on Estimating ReHab Costs http://www.biggerpockets.com/flippingbookYou might consider Niche or Specialized Housing like student housing.

Ive went threw twice, set up a business plan for the 3 rentals, had a professional heating and plumbing company go threw and give me an estimate.

Grab 3 estimates from local contractors for repairs, lets say ~25k.70% ARV - repairs puts the cost to the flipper at 45k.  

Scott The Book on Flipping Houses, The Book on Estimating ReHab Costs http://www.biggerpockets.com/flippingbookTo find out about an area go to IREM.org search for ARM certified property managers.

Our 2ndary offer is, as you suggested, a purchase contract with a contingency on the moocher being removed AND the rehab estimates being no greater than our worse case projections, since we can't get inside it.

I would just recommend using the GC and get several estimates unless this particular GC you know. 

I've been pleasantly surprised by the passion and diligence of so many using this tremendous resource.I've bought the book on estimating, and have started it, but obviously vis a vie this question haven't completed it yet.

We have built both companies on intergity and that's one principal we stand firm on, we have licensed contractors to give accurate estimate cost for repair and an offer an home inspection to give the buyers the truth about the home that they are interested in.