I just started climbing and I am wondering if it's safe to use my body harness and the 2 carabiners from via ferrata kit - they are 25|10|8KN resistant - for climbing too.
Yes. As stated by @Charlie, the ratings on the gear is more than adequate for safety while climbing. The reasons to buy a climbing harness are weight, comfort and convenience rather than anything to do with safety.
Use your own financial situation and importance of the benefits to decide if you should buy a dedicated climbing harness. If you climb once a year, its hard to see how you could justify it, if once a week, a dedicated climbing harness would be a worthwhile investment. The only caveat - if at any time using the wrong gear distracts you from the enjoyment climbing, you need the right gear.