What needs to be replaced depends on what is wrong. Does the system leak? Replace the seal or component that is leaking.
Compressor wore out replace the compressor AND check the orfice/screen for wear debries. If signifient wear debries are present entire system should be flushed (not an easy task) or replaced (not cheap).
Although it might be difficult to find an independent garage willing to handle R12, I found a local salvage yard that also does airconditioning work. They sell the recycled R12 for less than they sell R134a. I think it's $30.00/lb of R12 vs $40.00/lb of R134a installed. When I asked how they can sell it so cheap they said it's because they are required by law to remove the R12 from all cars they take in. Check your local salvage yards and see if they can offer a similar deal AFTER you make sure you don't have any leaks.
If your accumulater is leaking (that was the trouble with my "94 last year) auto zone has them cheap enough. I think I paid about $50.00.
