Hi Daniel, not to dampen your enthusiasm for diving into a restoration, but have you considered passing on this one? Meaning: this isn't your restoration candidate, but it may be a good parts car to use to restore another 850 with a body that is less rotted?
Years ago, I bought back my first car with the intention of restoring it. It was a 1980 BMW 320i that I bought from my uncle when I was in high school, and my sister drove it after I moved on from it. When I got it back from her, my intention was to completely disassemble it and restore it - I wanted my first car back for nostalgia reasons! Once I had it disassembled (all parts tagged and bagged), I had a local auto-body shop owner stop out and give me his assessment - I trusted his opinion, as he had performed various accident repair work for us and his work was top-notch. Once he looked over the shell and the amount of rust rot in the trunk, floors and elsewhere on the body, he gave me a sobering assessment: he said "You may not want to hear this, but it's not worth saving. You'll never get out of it what you'll have to put into it." It took a little time to digest that feedback and give up on my idea of restoring my 320i - my first car - but ultimately I came to realize he was correct.
I came to realize that it wasn't just a financial issue - restoring this car wasn't about flipping it to make money - but that the amount of time and money I would invest on this car could be used more efficiently. Maybe on another restoration project that was a better starting point, so that would mean less money and time would go into a car to reach the same endpoint (an enjoyable, restored vehicle). It is also important to be responsible with your time, not just your money. You can't earn or buy more time (with family, friends, etc.).
So, maybe this 850 has opened the door to a project, but this body isn't the candidate? Maybe get the motor/drivetrain sorted out while you search for another restoration candidate? Find an 850 Spider with a better body, but a blown engine or transmission - that could be a reasonably inexpensive option? And then you'll already have a stash of good parts salvaged from this one to set it right.