30-year-old attic condominium evaluation / risks

No, that's not true. It is legally regulated in the WEG - windows are ALWAYS common property. This cannot be deviated from in the declaration of division or community rules, or any deviations are invalid.
*Smart aleck mode off*

It can get interesting if a resident goes rogue. In the worst case, the new windows would have to be removed again. Hence my note.

All other owners have already replaced the windows at their own expense. Whether it is necessary would certainly need to be checked. Of course, the appearance and material would also have to be agreed upon within the WEG.

For a building from 1991, with a reasonable location and decent demand, I would absolutely do nothing to the apartment.
Because an expensive renovation cannot simply be written off immediately but may be considered as subsequent acquisition and production costs. Better to do a new bathroom or new doors or new flooring when a tenant changes in a few years. You can then deduct these expenses directly as advertising costs and ideally get up to 42% back on taxes.

The apartment would be owner-occupied, not rented. Therefore, I could only declare a maximum of 1,200 EUR per year (20%) for craftsmen services in the tax declaration. Additionally, there might be a 15% KFW subsidy for windows or 10% for the bathroom. But that would be sufficient if everything else with the apartment is fine.

All other owners have already replaced the windows at their own expense. Whether it is necessary would certainly have to be checked. Of course, appearance and material would also have to be coordinated within the WEG.

Doing things is one thing, being allowed to do them is another. Of course, in a small WEG it's easy to just talk to the neighbors. With 30 units, there are more likely to be troublemakers.

The apartment would be owner-occupied, not rented out. Therefore, I could only claim a maximum of €1,200 per year (20%) for craftsmen services on the tax return. In addition, there might be a 15% KFW subsidy for windows or 10% for the bathroom. But that would be sufficient if everything else with the apartment is fine.

With owner-occupation, you are exclusively bound to your own wishes and your wallet. For example, if you repaint, I would convert single sockets into double ones or create a new slot for an additional socket. If it doesn't get warm enough in the living room during winter, I go to the hardware store and buy a bigger radiator (inform the property manager or heating meter reader so that the billing is adjusted, etc.). You can retile the bathroom yourself (if you want) or cover walls, where you don't want tiles, with Renoboards (thin plasterboard) and just paint them.

Windows are ALWAYS common property.

Yes, but the repair and maintenance can be charged to the individual owner through a partition declaration. That is probably what is meant here.

I would definitely look under the paneling of the bathroom, because without a window mold could have accumulated there.
The layout of the apartment is quite nice. What orientation does the loggia have?

Thank you very much for the hints and advice.

I also learned from residents in the WEG that previous tenants had problems with drafts.

Yes, but the repair and maintenance can be attributed to the individual owner through a declaration of division. That is probably what is meant here.

I would definitely look under the paneling of the bathroom, because without a window mold could have accumulated there.
The layout of the apartment is quite nice. What orientation does the loggia have?

The loggia faces south. Thank you regarding the hint about the bathroom. Unfortunately, you cannot remove any boards before purchase. I would have liked to see underneath myself.

Otherwise, I also learned that previous tenants had drafts in the apartment. Where they came from could not be said. Checking that more precisely seems almost impossible in the buying process. If it is the windows, it would be correctable. Is there also a real chance that the vapor barrier has leaks in such an apartment built in 1991?

Where did the draft come from? Window or roof? If the boards cannot be removed, then work with a mold detector.

Where was the draft coming from? Window or roof? If the boards cannot be removed, then use a mold measuring device.

So far, I have not been able to find out where the draft is coming from. I received the information from neighbors that previous tenants had difficulties here. The most likely cause would be the windows, as they are 30 years old. I will try to get more information.

Otherwise, there were no noticeable odors in the apartment. Would a defective vapor barrier be common property or to be corrected at one's own expense? Presumably, it would depend on who caused the defect?