Best way to install AC in my 2000 Echo?

The issues are parts and labor. Most likely need to take apart the dash to install the evaporator and controls. Plus run the tubing. Then the system must be tested, cleaned, purged and charged which takes some special equipment.

Unless Toyota sells a complete kit you would need to run down part numbers, etc. If they do have a kit might check the price and have an A/C shop quote an installation price. They may have lower labor rates then Toyota. Or maybe they can get the parts. Remember you will also need to get any electronic control systems and tie into the cars idle up system. This raises the idle when the A/C is on to keep the engine from dieing. Plus any extra coolant fans as the A/C requires then and their control system.

You could do it yourself but read up on it and what special equipment is required. One item is a vacuum pump.

Another alternative is to find a junk car with A/C and remove every A/C related item and transfer them to your car. Then take the car to a place that can test for leaks and charge the system.

The best bet for information would be a Toyota service manual.