I'm much further south than Pyeongchang, but the wind chill was in the teens (F) this morning, and it's only November, I'm currently sitting at 1,500ish feet.
How expensive, compared to the USA, depends, of course, where in the USA...and where in South Korea. Seoul is easily comparable to New York, Washington, D.C., or any other major city, and I'm sure for a huge event like the Olympics, Pyeongchang will also see those sorts of prices. But if you're a jet-setter that likes hitting all the major cities, it shouldn't be too much for you. If you're from a small town in Mississippi, the prices might blow you away.
Travel adapter, South Korea uses 220VAC, 60Hz. Most laptops, cell phones and other appliances that use an external power supply usually accept voltages ranging from 110-240VAC and 50/60Hz, but I would recommend checking each of your power supplies individually. If you're bringing anything that requires 50Hz, don't bring it (particularly surge suppressor, I found out the hard way on this one once). If you want to bring anything that takes (nominally) 115VAC, it would require a step-down transformer, which can be purchased in many places, but may be an expense that just doesn't make sense for just few days. But, South Korean outlets are the two prong outlet/plug. View this link: http://www.wikiconnections.org/power-su … er-outlets
South Korea is well-known for being one of the best connected countries in the world. Free publicly-available Wifi is all over the place, and it's usually lightening fast. That being said, I have a SK phone with a pretty decent data plan, so I mostly use that, since I'm averse to using public wifi (the IT security guy in me).
Good to eat?! Bulgogi! Everywhere. Beef and leaf, as many Americans might call it. I love Bibimbap (rice salad), usually prepared "dolsot" (hot stone bowl), with an egg. Buchimgae (Korean pancake) prepared lots of different ways. Kimchi...all sorts of kimchi (I particularly favor the radish kimchi).
I hope I satisfactorily answered your questions. South Korea is a VERY safe place with very ethical, friendly people. Granted, this is a generality, since there are bad eggs everywhere, people are all individuals. As long as you are prepared for the cold weather (I can't imagine it being colder than England), you should have a good time.
