RetireJapan

One thing to check on is fees (and that there's nothing in arrears). Ones I've seen are 修繕積立金 and 管理費, but there may be others. For those combined, the newest buildings tend to be just under ¥20k/month, while older buildings can easily be ¥30-40k/month, and I've seen fees up to ¥50k/month for large, older units.

These are tied to floor size (sq. meters), so larger units will cost more than smaller units in the same building. Older buildings are more likely have supplemental fees for roof work, elevator repair/replacement, etc. Sometimes these are a one-off (a special assessment all at once), sometimes it's apportioned/collected monthly. You should check to see if there has been recent work done, or coming work scheduled/discussed, along with how that gets funded.

Edit: also, I once saw a place that was nice, but while it started life with gas (cooking, hot water), it had switched to electric for those things, since re-doing the gas plumbing was probably too expensive. Something to consider.

You should also look at how much money is in the repair fund, and what kind of work has been done on the building.

The biggest danger is probably ending up one of the few remaining owners in a 'ghost manshon' where most of the units are empty and not enough people are paying the monthly fees. This is probably much less likely in Tokyo though...

To add to the above,

Most important point is that a mansion build before 1982 will not be conform to the latest earthquake norms (except if it has been retrofit). You have to understand the risk attached.

Second is that the other owners could decide to put the mansion down and rebuild it pretty soon (tatekae 立替) The decision must receive 80% of the votes from the owners. If it does you have to pay the associated costs even if you voted against it. It works the other way too, you might want to rebuild in a few years, after an earthquake damaged the structure for example, but a majority might vote against it.

Last is the insulation and soundproofing of the mansion. I would check carefully which renovation your unit went through as this would drastically impact your comfort. Insulation can be added for a reasonable price but soundproofing is more difficult.

As recommended above you have to very carefully check the history or repairs and the repair plan of the building. I would not take the risk if your Japanese is not up for the task or if you cannot find a trustful adviser.