Level 3 RICS survey or structural engineer

Hello, I'm in the process of buying an old house and all the changes there were done without building regulations or planning permissions. And of course, the seller has no documentation of any type (calculations etc). We are talking about a loft conversion, chimney breast removal and more. Level 3 RICS survey came with some minor problems, but nothing structural.
Do you think I need to bring a structural engineer to view it? Will they be able to say if anything is safe without open holes? I don't want to pay more money for someone to just tell me "I don't know, I need to open wall/floor/ceiling".
I wonder if the way to go is negotiating the price based on the risks and cost of regularisations, or if I really need to bring an engineer to have a look even if beams and steels are all hidden.
The RICS surveyor didn't notice anything wrong, but he also didn't flag the chimney breast removal... I was the one to pick it up looking at the floor plan later on.
Thanks a lot!