How bad is this stuctural/foundation issue?

[Domenic Matesic]

Hey all! We looked at a potential investment property yesterday in NJ and found the back of the house to be sloping down significantly. Maybe the house was extended at some point, but not supported properly so eventually it sank, and the owner chose to ignore the foundation problem and finish the inside anyway- the floors inside look new but there is a very noticeable slope. 

I attached some pictures of the inside/2nd floor (house is 2 stories) and the crawl space.

We want to go back to the property with our contractor to get a better sense of what it will take to fix, but I know we'll also need a structural engineer to take a look at some point.

My questions for anyone who has more experience with this sort of thing:

1) Looking at these pictures, any initial thoughts of how bad it is?
2) If you were pursuing this property, is there anything else you would do before making an offer? This is a foreclosure and we are not getting much info from the realtor, maybe there is some digging we can do with the town? Any other ideas to get a better sense of how much of a pain/cost this will be to fix?
3) Does it make sense to bring in a structural engineer for a formal assessment when we are under contract, during the inspection period? I'm assuming they will be pretty expensive.

2nd floor inside

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crawl space - back of extension

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crawl space - back of original house