Based on what you said about yourself, I rec that you save up a few hundred dollars, and find a pro who works with r12. Have them replace the receiver/dryer & pressure switch, evac the system, recharge with r12, and be done with it in a few hours.

Or, use the "Search" function above to find and then read the many tens of threads here that have already answered your questions

You should evac the whole system. You should not just add r12.