Buying co-op with extensive renovation w/o permit?

We are interested in purchasing an apartment in an 8-unit co-op in Center Slope. The apartment was extensively renovated about 5 years ago — interior walls moved to open up space, bedrooms reconfigured. We discovered that this work was done without a permit. At least one other, I think two apartments in the building have also had major renovations without permits. We did a pre-purchase inspection with a PE and he gave the building an “as expected” rating with some typical issues but nothing significant. How concerned should we be about becoming liable for this situation in the future? The building has no C of O. It’s old (1800s) and some have suggested that future work (needed repairs or other renovations) could cause the DOB to become aware of this past work and require either a retroactive filing and/or get a CO across the whole building. What are our options? Might we request that the sellers file the work retroactively? I know little of the building code but it seems it might qualify as Type 2 (I don’t think egress was changed) and could be filed and certified by an architect without DOB inspection? On the other hand there’s a small second bedroom / child’s room, and it’s possible it might not meet code as a bedroom, which could have other ramifications for filing? Is this a process that could take days, weeks, months? Do renovations like this usually necessitate getting a CO issued if the building hasn’t previously had one (there is an old i-card record but it is mostly empty and doesn’t have any useful info on it)? Trying to figure out how the rest of the co-op would react to this situation, beyond the current owner. Thanks so much for any advice!!