Moving in July: Need an Apartment for 6 Months? (Mexico City)

Hey!

I'm moving to D.F. in July with a close friend. We're looking for a 2 bedroom in Condesa, Roma, or San Miguel Chapultepec for 18,000 or less for a 6 month term.

How easy is it to get a 6 month lease?
Do I need any special documentation to sign a rental agreement?
Where should I look for the apartment?
Do you think our budget is too low?
Is there anything else we should keep in mind?

I'm excited and a little confused about where to begin. Any advice or info resources would be greatly appreciated!

Thanks!