I really don't think steam cleaning will do the job of removing mould, so it's likely the interior will need to be replaced. Although this may put many prospective owners off, you must remember that these cars are over 30 years old now and the interior materials were never designed to last that long anyway. Nothing better than the sweet sent of that new upolstery and carpets too! At least the dash hasn't cracked!
Spheres are just a maintenance item on a 6.9 and I'm sure it wouldn't be that hard to trace the source of the leak down / slow pump up problem.
Rust is often a problem in these cars too, so all owners are well advised to seek a mate who does good panel work!!!
If I had the money and space, I'd buy it for the first aid kit and the manuals alone!!! Haha!