American Foundation School in DF - Good location to look for an apt?

Hi, I will be moving to DF with AFS and need a little advice on where to begin looking for an apt. I "should" have about 750 US to work with in stipend toward rent... I REALLY don't want to commute more than 40-50 minutes - less would be better. The place can be small but it MUST be clean, safe and in a location with plenty of transportation, restaurants, supermarkets.People say the school is centrally located... Really? I have looked at a map and it doesn't seem that central to me. I have  been to DF several times and "central" to me is  Condessa, Roma, US/Japanese Embassy area. Centro Historico is "central" but do people live there? From the map, it looks like Roma would be fairly close. It also looks like it is near the Observatorio bus station but terrain in Mexico being what it is, it could be close but you would have to climb a mountain to get to the school from the station.  So, just looking for good ideas of where to look for a convenient location for an expat to live in comfortably and commute reasonably.  Thanks for your help.