If you are using chlorine to sanitize your tub, you should buy the K2006 kit. If you are using bromine to sanitize your tub, you need the K2106 kit. Both kits have tests for your pH level, total alkalinity, and calcium hardness (for balancing your water). and then a test for your sanitizer level.
I bought my first kit (K2106) on amazon for around 50 bucks. I have since switched to the dichlor/bleach method, and have bought more test solutions from amato industries. As some of you know, I really enjoy testing my hot tub water. I have piece of mind that my water is in balance, and my family won't be getting a rash from using the tub. Crystal clear water is standard at my house, because I trust the test kit is giving me positive feedback.