/// The repair necessary to convert is determined by the manufacturer of the car/AC system. Some brands of AC may require replacing the dryer element. Be sure you got a quote for the correct brand.

The quote may include replacing hoses also, depends on what is on there now.

Each refrigerant has a unique set of fittings. They will not interchange. The reason is to protect the required recycling of the old gas. Stick to what is being used in your location. If you use a cheaper system and DIY, at a later date you may find that no one will work on it because they do not recycle that particular refrigerant.

In addition what you use will effect the resale price of the car. Conversion is meant to be one way, one time. It is necessary to flush the system with a solvent and replace all fittings to switch later.

Go to the EPA.gov site and search the site for the details of the Clean Air Act, Montreal Agreement, and all the chemical, and physical data. This includes the size and threading of the fittings. This information is in HTML pages and you can click from one to the other.