(budget 50-70k ~ which includes demoing  a POS double wide on the front half of the property). 

The seller is under no obligation to disclose unpermitted work unless you ask.The exception would be if the seller filled out a "Seller's Statement of Property Condition" which is normally attached to the listing, (which specifically asks about unpermitted work) and didn't disclose it, you might have an out.If the seller goes for a retroactive permit, the town could make the seller demo all of the unpermitted construction and start over.If you're already under agreement and there's no provision in the offer or P&S to address this, the seller has no motivation to go for the retroactive permit.  

If you know this then you should be able to get yourself in the ballpark adding some fluff to the estimate.Are you just replacing cabinets, flooring and counters or are you going wall to wall demo and rehab?

Their contractor dissappeared on them after doing little more than the demo and now the owner is looking to sell it to me for a hot price.

The other option is to demo and build condominiums if you can get the plans approved.

Square footages and prices starting on the left were:1,700 sq ft- 385k1,900 sq ft- 399k2,050 sq ft- 440k2,100 sq ft- 450kTotal development time took approximately 3 months to get demo complete and lot lines approved.  

Yes I was one of his demo guys who went in prepped the homes for remodeling. 

They will all do demos and let you know what they do better than the other.  

Some businesses might be willing to share their experiences, or consultants2) Have you considered setting up a demo with a popular AppExchange app -- propertybase.

Demo started on May 22nd and took 45 days to completion.