The currency is the mexican peso. $1 is equal to about 19.77 mxn. I would travel with some cash. Easy to get at the airport.
Oaxaca is known for food, culture, ceramics and textiles! Please see the following articles: https://www.cntraveler.com/story/inside-oaxacas-female-led-craft-revolution https://www.nytimes.com/2018/10/12/t-magazine/san-marcos-tlapazola-pottery.html https://www.cntraveler.com/story/why-everyone-will-be-traveling-to-oaxaca-this-year
As far as crime goes, Oaxaca is one of the safest states in Mexico. Oaxaca City, in particular, when compared to other urban centers, retains a friendly, small-town vibe.
Will keep you posted as we get closer.