I must be missing something.
it's a multifamily building, so I assume(...) it's a 5 unit or more. So the value of the home is based on the NOI, right?
so, if the cashflow(NOI) is there, the value is there. When I'm looking at deals, I don't care what it costs, as long as it has a positive cashflow.
Your question is, how to negotiate with the seller. I agree an agent[broker if you haven't done this before. One strategy I've used, on a small scale, is offer full boat, then come down as unexpected cost arise in your due diligence process. If you're in a small town, this may give you a bad name though.
Or, find out everything wrong, give them cost to repair, they know it's a lot, and make a presentation to them. Here is what you're asking, here are the cost, this is why I need it @ my price. Work up from there.
You also already know what the seller wants. Do your best to find their break even point. Use a amortization calculator to figure out about what they still owe, and see if that number works with your numbers.
Good luck!