It would help if you can specify the street/nearby tube station.
PAP is a website for offers from private people (not property agencies). There, prices can be anywhere from reasonnable to out of market (emotionnal pricing).
You will have to compare with websites used by agencies, such as www.pro-appart.com or agency listings and do a search on what's now available on the market and comparable to the flat you are considering.
Reviews/pricing on magazines only gives you a range and do not reflect what's the + and - of the building you are considering.

There is a lot of laws to protect the buyer in France. The Syndic (property management) have to let you access all documentation over precious years regarding what work has been done and what is planned for the next year.
What decision has been submitted for vote, voted for or against by other landlords. It will give you a sense on the how easy/difficult it will be to get approval for the renovation work you plan - should you need to modify the building outside appearance (rooftop glass?).

If central heating and you need to work on it, you will have to plan it with the Syndic and probably use their authorized contractor.

Evaluation: you need to visit the flat if you have not been there - as well as the area near the flat to decide wether or not it is worth it but 6th is indeed prime location.
The higher the floor in old building, the lower is the ceiling.
Last floor was usually for maids/service room, thus check if lift goes till that floor or the floor under as well.
It is a good thing the roof has been checked and renovated recently.
But agree, quietness and view is a big + in Paris be it on a quiet street or back yard.

Closing cost (Notary): there was a modification of cost in the last month and it should be close to 7 to 8%.