The lower bolts can be challenging, but if you are a good backyard mechanic, you should have no trouble replacing the compressor.
R-12 refrigerant was the bad environmental actor, replaced by environmentally "friendly" R-134a. If you are still concerned about releasing even R-134a into the environment, take it to a shop with a recycling machine. They will probably discharge your system for the value of the refrigerant they recover. They are more likely to do this for you for R-12, as it is much more valuable. But even if they do charge you, it should not be much.
You will need to replace the oil that you remove with the compressor and/or accumulator. You want to make sure the replacement compressor is well lubricated before you install it. I usually squirt refrigerant oil into both compressor ports and rotate the shaft by hand until I can hear the oil sealing inside as it pumps.
It would be a good idea, once you have opened up the system, to replace the orifice tube. They are only a few bucks. And, after it is all back together, you will need to pump the system down with a vacuum pump for at least 30 minutes. I like to do 45 minutes, as I have my own vacuum pump, and that removes sufficient moisture that I can avoid changing out the accumulator. Most shops routinely change out the accumulator, because it reduces the required pumping time for the removal of system moisture.
If you do not have access to a vacuum pump (local rental shop maybe?) you will need to pay a shop to pump it down. Check for leaks in the system by closing off the valve to the pump and watching that the vacuum/low pressure gauge needle does not bleed down over 15 minutes.
You can recharge the system yourself with hoses and cans from Wal-Mart. The first can can be charged into the evacuated system up-side-down as a liquid with the engine off. To be safe, the remainder should be charged as a gas with the can right-side-up. I drive the car down the road to get the last of the recommended charge into the system. The air flowing through the condenser and higher engine speed helps the system to equilibrate much more effectively than anything you can do in your driveway.
There will be a sticker in the engine compartment that tells you how much refrigerant to charge. Do not exceed that value. It is usually about 2 pounds for a full-size system. Three cans of refrigerant total 2 pounds and four ounces. But when you open the can, you should purge the charging line by releasing some refrigerant until you are sure all the air is flushed from the line right before you connect the hose to the low-side charging port. This may take an ounce or so. That will give you just about 2 pounds from three cans.