It really depends on how serious you are.
How much are you training?
Are you very very very serious?
I can't emphasize very enough, back when I was "good" enough to compete professionally I trained extremely hard, and as dedicated and good as I was, I wouldn't stand a chance when looking at some of the competition today.
What's changed is that although I was very good, I could have stood out alot more back then and broken through to the professional ranks because of it, today I wouldn't stand out at all.
Another thing that bothers me about your question is that you ask about a different number of combat sports, K-1 and UFC are very different, if you are looking for which would be easier to break into then you have it all wrong. There is nothing "easy" about combat sports, you have to be willing to give it your all, and then some.
Having said that, looking for the easiest one to get into would indicate that you have no clue of what would go into getting to that level.
Now if you are serious then I HIGHLY recommend that you join a known gym where you can train with known fighters.
You'll probably have to undo some bad habits you might have picked up in your kung fu, karate training, this of course depends on several variables.
Either way you'll have to dedicate the majority of your time and energy to training, conditioning, and diet and rest to develop the skills where you can ever hope to enter the pro ranks.