Carabiners are definitely dangerous. If you are being dragged, even if only a little, you will not be able to get a biner unhooked. Period. Make damn sure you trust your QR. When climbing, you never want your harness to detach from the rope. When kitesurfing you want to be able to detach yourself from the rope (bar) at an instant's notice. Climbing gear is designed to never accidentally release, even if it is difficult to release when you intend it to. Kiting gear is designed to always release when needed, even if this means it releases when you don't want it to. You can not release from a carabiner that is under pressure, they are designed this way. Your primary QR should negate this fact, but they sometimes don't work as well as they should.
Some people used to ride with a snap shackle, occasionally with disastrous results, you could try that, but by the time you've bought a good snap shackle, you could have just bought a used harness on ebay. If you know your gear inside and out, have tested your QR, have the experience to deal with a kite dragging you in the event of not releasing, and think you can ride with a biner, great. If you are a beginner looking to save a few bucks and think you can safely ride with a carabiner, you are wrong.
Breaking wouldn't be a concern, as climbing harnesses are almost certainly held to higher standards than kiting harnesses. I had a very similar harness back when I used to climb. It was uncomfortable as sin, and when climbing you don't have weight on your harness for hours at a time. And you aren't submersed in salt water.
If you do decide to use it any ways, then clip in as if you were climbing- through the belay loop (the loop that goes through the waist and leg straps) OR through both the waist AND leg straps.
Will it break? Almost certainly not. But I wouldn't be surprised if you chaffed your balls clean off. And I wouldn't be surprised if you had an accident relating to being unable to release your kite.
I get it. $200 for a harness is a rip-off, but spend some time on ebay looking for an old kite harness. It will be worth the $40 you spend.