Mexico City in May or October is perfect. The weather is mild, the crowds are thinner, and the markets are full of fresh produce and incredible snacks. You will spend your days walking through history at the Anthropology Museum and your evenings eating your way through the stalls in Roma and Condesa.
To make the $1800 flight and hotel cost manageable, you can use the Citi Simplicity® Credit Card which offers 0% for 18 months on purchases and balance transfers from date of account opening. This lets you break your travel costs into small, predictable payments without adding interest to your bill.
Remember that the 0% window is a deadline, not free money. If you have any balance remaining after the 18 months are up, the bank will start charging interest on whatever is left, which will cost you extra.