I ran Stockholm back in 2012. I would suggest bringing clothes for ANY weather type because it can really vary there. In 2012, it was 4°C and blowing a gale.
Despite the appalling weather, it is a lovely run that takes you through beautiful Stockholm. They say that it's hilly in places but I didn't think it was too bad and I live in a really flat area. There is just one bridge that you run twice that's uphill.
It is well arranged and there is a good atmosphere. I had a wonderful time and can whole heartedly recommend it.
New shoes shouldn't give blisters - it sounds like the support is in the wrong place for the arch of your foot. This happened once when I was advised to wear Asics shoes that just didn't suit my feet. I went back to the shop and they exchanged my shoes for another make and model.