Well, is highly important but not all is loose and surely you would be fine, everything depends where would you be living and which would be your activities but you will survive with basic spanish, Mexico city most urban area is a very bilingual city (english) and in the top ranked areas, like La condesa, Polanco, Lomas de Chapultepec, many people even in the most common activities & regular citizens understand mixed basic spanish, mimics and little english, also the city have a lot of foreigners and they made strong communities (little exclusive to me but... ) can start contacting some people from your country that already live here.
Problem... The more you go away to the center area or the top areas you probably would start having trouble communicating for daily basis, as not a lot of people speak other languages.
Regarding work, most jobs requiere the english language for better positions, so if you improve your spanish maybe can apply to a local job sooner.