Bulgaria is in the EU since 2007, so you dont need some special documents or visa. Prices for a holiday is usually - 20EUR / 17£ for a hotel room per night and usually the same for eat out for a day. There is few ways to get from the airport to Bansko, Pamporovo or Borovetc - one is to take the bus from the bus station which will be the slowest and dangerous one and to take a Sofia International Airport transfer
That bellow is one of the Bansko ski slopes :