I have no problem with hiring cheap help if you can get it I am sure there are good people that need the money. I don't know what is like there but here if a home owner hires people like that he is responsible for there workers compansation and there safety, thing like scaffolding and harnesses and safety ropes, proper dust masks to deal with old insulation and the mold that you say is there. As this is a one time time deal they would ask for one qualified person on the job and a complete plan of action and proceedures. It may be cheaper to find a contractor with a sharp pencil.
I understand you like a flat roof but that is a big remodel and I would look at all the other options also. I would get quotes from a couple mold removal companies, and see just how bad that is. As you said the soffits are not vented properly, with them removed it may be possible to slide new rafters in along side the old ones.
These are the quotes you would need to go after the seller anyway and then you could decide whether a flat roof is worth going for.