Sagging floors but cause unknown, should I go through with deal?

[Umair Ahsan]

I have a duplex under contract here in DC.  The floor of the first floor unit is dipping in and many of the corners aren't quite 90 degrees.  All signs of floor joist rot, damage or settlement underneath.  There is no basement or access to a crawl space underneath so it is not possible to tell the extent of the damage or get quotes on repairs.  It also makes it harder to repair as the floor must be torn up to access the joists underneath.  The contract has an as-is inspection contingency so I can still get out of it.  I may be able to negotiate a credit but not sure as DC is a sellers market.  The second floor unit looks just fine though, and the first floor unit sagging is isolated to one side, likely damage to just 1-2 floor joists.  

I'd appreciate any advice.  There is a bit of a risk here.. can't quantify the repair cost... what would you do?