You might want to check out Amerispan.com for Spanish lessons. They have a one-week program in Playa del Carmen for about $500.00 per week. I think that includes the homestay. I loved their program in Guatemala.
I suggest long skirts and blouses with sleeves for travel on busses, in towns and remote areas in Mexico. Don't take many tops with you and buy some blouses there! They are light-weight, beautifully embroidered and they dry quickly. You can find them everywhere. Oaxaca is famous for a particular style of embroidery. I'd suggest you buy only one in each place and look for other styles as you travel. They make very practical souvenirs!
Que se diviertas mucho en Mexico!