What's the best way to create an IoT services startup?
It's okay to use a development kit?


It's totally up to you and your project requirement. Most of the development kits have almost all the features available for its Microcontroller and you may not be using all those features in your project. Development kits are best for testing purpose. You can write your demo code and test on the kit to check if it works or not. It is always best to design your own PCB. So chose any Microcontroller first which satisfy your need and then decide which development kit to purchase for it. Use their schematic as reference to design your own PCB. 


  What are the best kits for this kind of project...


Selection of MCU and its development board completely depends upon your project requirements. You can have a look at Microchips development board. You can go with Explorer 16 board and PIC32 Ethernet Starter kit. Microchip provide its TCP/IP library for these boards so you can directly build and upload their code on the board and have a demo  app working for you. Later on you can understand their code and can do necessary changes to include your own files. Here is the link for Microchip Libraries for Application.