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.
The carabiners are definitely good to go as those are pretty standard ratings. Most climbers using climbing harnesses and not body harnesses, I would suggest that you buy a climbing harness as I think that it would work better and be more comfortable for you.