I can only address your concerns about the food and water quality, and I can assure you that they're completely safe.
I live in New York City where the tap water is very highly regarded, and I don't consider Seoul's to be any worse. Same goes for the street vendors, etc. (though, even if you were back at home you would avoid anything that looks questionable, yes?).
I hope this helps. You can probably do some Googling to find additional notes on food and water safety, as well.