I am a Kung Fu student, planning to enter a Karate competiti

[1st KYU]

every tournement ive been to calls for no kicks below the waist.

and you wont need a karate uniform or belt, choose a division which you think is of your skill level.

[CA-Isshinryu]

Many open tournaments base their divisions on time traning in your style and age. That takes the belt thing out of the picture. Also, wear what you normally wear. Most of the time you are called for your kata and sparring right after each other so there is no time to change so wear the appropriate uniform.

Read the rules before you go. You might need special protective equipment that you do not normally use. If you do not have these then you will not be allowed to spar. If you need extra room to perform your kata then you need to let people know. A Kung Fu person came to one of my schools open tournaments and he needed 2 rings to do his sword kata in. It was quite entertaining to watch.

As said above, no kicks below the waste. Many tournaments do not even allow sweeps of the feet. Usually for medium to upper belt people the basic rules are light contact the body and no contact to the head.