In the UK, Fords and Vauxhalls are still cheapest for parts.
Tax depends on the age of the car and the CO2 emissions, and is such a small cost relative to the cost of the car that it probably isn't worth taking into account. Kia and Hyundai cars tend to be about average compared to direct competition for CO2 emissions, so the tax is about average too. Remember that a larger car will have greater CO2 emissions, so you cannot compare any Kia or Hyundai against any Volkswagen - it is only relevant to look at two directly competing cars, such as an i20 1.0T vs Polo 1.0 TSi.
The big advantage Hyundai and Kia have over the competition is the warranty. Kia's warranty is 7 years or 100,000 miles, which is the best available for low to average mileage drivers, while Hyundai's 5-year unlimited mileage warranty is the best available for high mileage drivers.
I have a 2010 Hyundai i30, which was bought new. It has been perfectly acceptable over the past 7 years and 85,000 miles. It hasn't been perfectly reliable, but much better than the 2003 Renault Mégane that preceded it (although not as good as my 1997 Rover 200, which was faultless apart from a couple of bulb failures for the 5½ years I had it).
I would also add that I bought the car from, and have it maintained and repaired at a family-owned Hyundai dealership (rather than one of the dealer groups) and their service has been excellent.
Insurance is based on a system of 50 groups, (there is good information on this at the Parker's website, linked below). Again Kia and Hyundai cars are about average compared to their direct competition.