As an investor, I like to stumble upon properties with these kinds of challenges, because most retail buyers run away from them as fast as they can.
I agree with Karen: you and the seller need to do some more due diligence. You both need to determine (via inspection) what if anything needs to be torn down, are the modifications up to code, are there any special use or varience issues at play (and will they transfer to the buyer), how these changes might affect the buyer's tax basis, etc.
Instead of asking the seller to pay for the extra inspections, I'd make arrangements to order them, get cost estimates, and ultimately use all of this to negotiate a lower price. I'd also make it a point to disclose all of the issues discovered in writing, and remind the seller (or his/her agent) that henceforth the seller will have to disclose all of them upfront if s/he opts to not sell to your buyer. (I only do that to play hardball on properties with issues where the seller appears to be trying to hide stuff.)