Here is what I would do:
Plan to replace the foundation, and fixing the drainage issues outside. I would probably expect to put in a tile/french drain around the perimeter and also change the slope of the yard of the house. If water comes in when it rains then you ALREADY have a major drainage problem that has to be fixed, and you have already provided the images of what the water damage has done. So it's not a question of how bad is it. It's a question of what it's going to take to fix the water issue, and the damage. It's entirely possible just fixing the water issue will resolve the problem and the house maybe fine for another 75 years. Also think about this. If you buy it live in it and its fine but then you go to sell it. What do you think the next guy is going to say when he sees that..... yup he is either going to walk away or ask for a price to fix it..... So you do it now or your stuck with the same plan later.
Your asking price should include the above as part of the plan. Work with your financier to create a plan to do the above and you can make it happen. If you can't get the finances in line with the expected plan then walk away. If you can do some/most of the labor yourself that should be "gravy" in your financial plan NOT the expected requirement. Things happen. You might get a new job... might hurt your back... etc. But if your financial plan includes the expectation of getting the work done then at least you'll have a roof over your head when the unexpected happens. We are all capable of amazing things under perfect conditions but having a clear plan that works even if you can't is where people get themselves into deep, deep trouble.
If you flinch at any of that walk away. WALK AWAY. If all of it sounds like fun. You can handle a lot of stress, physical labor, getting filthy dirty, and talking finances at the same time. Then you'll probably survive it.
