Take a walk through with the home inspector and see what he says. If you are still interested in the house, find a good general contractor (that you trust) and take him on a walk through. The contractor should be able to give you a good estimate on what it will cost to fix any issues, he may also give you some price breaks if you are willing to let him handle all the work.
Just make sure you are comfortable with the contractor before you ask them to go on the walk through, they might be upset if they take time to walk through with you and then you find a new contractor to do the work. Keep in mind, however, that if the contractor starts work and does a sub-par job, you can always fire him and find a new contractor.
Mold goes away when the water goes away, so unless your toilet is leaking again, you have some other source of water or leak on the upper floor. This could be any number of things, even things not plumbing related, like water getting behind the siding, or a leaking roof with water running down the inside walls. As far as killing the mold that is there, I wouldn't waste my time. If you don't correct the water problem then the mold will return very shortly.
I need to figure out if there's still a leak or some kind of humidity problem. Is it safe to assume that because the problem got worse there is? Should I try to find the pipes and see if they're leaking?
Since you have had issues with the toilet in the past that would be the first area I would check. If the drain pipes for your upper floor are leaking then I would expect to see discoloration bleeding through the paint on the ceiling and the walls. If this turns up nothing then in the hole you have already cut, I would get a professional moisture detector device like the one a plumber would use.
These can help detect moisture and humidity in the air which can give you clues if you found the potential problem areas. Don't buy a cheap one because they don't work very well, I would instead see if a rental place will lend you a good one.
I think I'm seeing some black wood in there (need a better flashlight!).
This doesn't necessarily mean anything, it could be discoloration from previous water damage.
I would still like to hire someone to fix the plaster ceiling. Is it going to cost a huge amount extra if I cut through the mesh and they have to replace some of it? Does area make a massive difference in price for that kind of job? For what it's worth, I'm fine having drywall put up instead of plaster.
How big is the hole that you had cut? You can sometimes repair a hole by cutting a small piece of drywall and fitting that back in place, then using some thinset or spackle around the edges. Sand and repeat thinset or spackle two or three times until you have a smooth paintable surface.
Best Answer
The pipe connected to the radiator has a deliberate space because it is really hot. Can air leak through walls that has mold? Don’t want to scare you but just about every breath you take may have a mold or fungus spores, in the Forrest 100% In most living spaces. Can you seal the pipes, I mentioned the radiator but there are special collars made to insulate and seal these, water pipes usually can be foamed to seal them. Make sure you know what you are sealing and use the proper material.