It's not advisable to have meals at stalls or untidy restaurants. 20 Florencia Ave. Chek the constantly changing rate .
Most banks offer exchange as well but they are usually more busy.
It is possible to beleive that Mexico City has a tropical weather, since it is located at the 19th parallel North. Distance from the sea shore makes humidity relatively low, and wind is moderate. All this conditions makes polution a major problem and unconfortable for those who just arrived but it is only at peak hours when you shouldn't stay outdoors. You shoud be prepared for rapid changes in temperature but also consider that the city inhabitants usually wear formaly. Short pants are not common and even odd.