I put my system in near 15 years ago. Had mentioned to a buddy in the business what I was considering and found out about rebates and cheep financing for power savings on top efficient units. Was a no brainer. He sent someone out to measure for ductwork etc. I put it all together and another friend helped finish and solder the connections. It was precharged. Forget the exact process of charging up the tubing between in and out units. Saved a ton over what I thought it would cost.
My thought on the 40yo unit is the money on refrigerant would be wasted. If not ready to bite the bullet on new system... Pop in a couple cheap window units to get by. You can sell them after you get a new system.