Heating gaze is going up

Hello friends, Since a couple of week I am facing a new problem in my car. I have noticed that heating gaze is going up and simultaneously coolant is overflowing from the storage tank cap. I took it to authorized service station, and after diagnosed it they tell me there is problem with 'Head Gas Kit' and need to change it. Thought possibilities of some work of head also need to do if any problem found there as well. The cost of 'Gas Head Kit' work around 20k and head work also need to do ...