I LOVE the older buildings as they have charm and character, usually have higher ceilings and generally constructed much better but the downside is possible plumbing, electrical and gas that eventually usually need to be replaced. I've owned some older units and I had the same thing happen where a gas line had to get repaired and renovated. Royal pain! Many years ago, there was an explosion with a faulty gas line and after that in several buildings they were going out and checking to make sure they were up to code. It was crazy but in several buildings they just shut off the gas and they were so backed up they couldn't get around for months.
It was horrible because it was a rental unit so I lost income on that for several months. This is one reason why many people prefer new construction. I had a client that also had a gas issue in his apartment and went several months without the gas company fixing the issue so he just changed his entire apartment to not use gas. Switched to electric hot water heater, electric stove, etc.
Also, so true about humidity issues! Typically you can smell it when it's really bad. In good inspections, the architect can usually detect it with equipment they have. I've purchased hundreds of properties in BA and typically when you can smell it, it means it's really bad and if you buy something you have to know full well that extensive repairs may be needed.
I was just retained a few months ago by an American woman that had humidity leaks in her ceiling for 10 years. She kept arguing with the building and they repaired it twice but then they ran out of money and never repaired it again. So you have to be careful with water damage. Always do an inspection.
Definitely the 3D walk through or video walkthrough is the best way to get perspective. Some photos are taken with wide-angle cameras and they look really big with no furniture but when you see the video or 3D walk through you see how tiny they are.