As of March 2019, 1000 Won = $0.88 USD. The easiest method is to approximate that 1000 Won = $1. So if something costs you 20,000 Won, it's going cost ~$20.
Google Maps does not work in Korea for navigation purposes. Please download Naver Maps.
Once you get the hang of the subway system, it's convenient! There are also plenty of taxis, and they're easy to hail. Even if you don't speak Korean, drivers can take you around (via GPS).
Honestly, there is a huge variety of good food, and good food can be found anywhere, whether it's a packed fancy restaurant or a hole-in-the-wall or a street stalls; some of my favorite memories of food in Korea is food from the streets.