I stayed in Marilo's apartment for 2 months and highly recommend it, especially for a longterm stay. The building is a bit older but the unit is clean and well cared for. The apartment has a very well stocked kitchen, a washing machine, extra sheets and towels, and anything else you would need for a long-term stay. I felt completely safe in the neighborhood as a female solo traveller, although it is worth noting that it is a bit removed from the neighborhoods where most tourists stay. The issues with smoking and the internet that other guests reported had been completely resolved. The unit does have a very small staircase, as other reviewers have mentioned, but it did not pose a problem for me at 5'8.
Most importantly, the apartment was extremely clean and when issues arose (as is inevitable during a lengthy stay) Marilo was extremely responsive and helpful in resolving them. For example, there were earthquakes during my visit and Marilo reached out to make sure I was ok and to give me updates on the building inspection. I highly recommend Marilo's place and wouldn't hesitate to stay there again if I were to return to that part of CDMX.