Don't do it. Use propane. Used in Europe and Asia. Not legal here but, if you're doing it yourself.........It's cheap and better than R12. Flammability is about the same. If you go with R134a you need more than just a "kit".
Which is usually nothing more than fitting adaptors. You need a larger evaporator core, larger condensor, different pressure switches, orifice, to get the same heat transfer rate. You also need to completly evacuate the system and replace the compressor oil. That's why most people are not happy with the results. It's like burning Kerosene instead of Gasoline.
You might be happy up north but, in the south..fagetaboutit!