4400 A\C

Going back to R-12 would be expensive just for the cost of the refrigerant alone (4 lbs @ $50/lb). R-134a is a lot cheaper ($6/lb) but runs a lot more pressue and is not as efficient. If your compressor is shot, get a Delco 134a compressor from your JD dealer as is already has the right synthetic oil preinstalled. The seals and other internal components are designed for higher pressures as well. The receiver/dryer MUST be changed and the entire A/C system should be flushed. The expansion valve should be changed and the hoses should be closely examined, expecially the hose that runs from the compressor to the condensor. This hose handles all the heat and pulsations from the compressor and will usually be the first one to go. If you can turn the rubber part of the hose while the metal stem stays still--that is a leak--replace the hose.

Another good way to find leaks is to charge the A/C system with dry nitrogen (200-250 psi) then go over every inch of the system with a spray bottle of kids bubble mix. You see a bubble, there is a leak. Wear eye protection and be careful. I would make this the first step so you could see how many things need replaced before you get started. $1000+ would buy a lot of ice and cool beverages!