The advice on the fruits and veggies is a great idea. I lived in Guatemala (Mixco area and the east coast) for a number of years. I never had too many problems. I ate with many families and learned to love the food there. I never bought from the street vendors, I don't do that here in the states either.
The water jugs are easy to order. One day when you see the delivery trucks stopped, go and ask the driver if you could buy some and have them deliver to you whenever you want. There are also small bags of water and fruit juices that are great for hiking and playing soccer or anything like that-very portable.
As for restuarants, don't shy away from the American companies, especially the closer you are to the capital. I ate at many of the fast food places, because I missed the American food. There are the regular hamburger places and Pizza Huts, Dominoes and Papa Johns. Also, right in the capital are some nice "sit down" American restuarants, such as Denny's or Village Inn.
But, there are also many restuarants that are of Guatemalan origin similar to those American restuarants mentioned above. These are just as good, sometimes better. They have a blend of Guate and American foods. Try them, too. Regulations for handling food is just as good in these places as in the US.
The capital city and surrounding areas are pretty modernized and they have many ratail shops and malls and grocery stores just as in the US. I enjoyed shopping in the tiendacitas in the neighborhoods to be a part of the culture. But, I also liked to go to the big grocery stores. There are two or three large chains that have everything you desire. Hiper Piaz is the largest and it is similar to a Super Target or Wal-Mart SuperCenter, just more on the food side and less on the other stuff.
I loved Guatemala, I would love to return. I was there until 2000. There are many places I never had a chance to visit and I am looking for time and money to return.
Hope this helps.