I agree with the information you have received. Should it be that you need a new compressor, and possibly even if you do not, I suggest that you switch from R-12 to R-134. The reason to switch is the cost difference - with R-134 the cost of a recharge is probably $50 or less. The cost of change-over should not be expensive either, when R-134 first came out it was expensive because it was felt that all seals needed to be changed along with the oil & freon. Now many shops simply vacuum the system, add the correct oil for R-134, and add the new freon.
It is extremely important to have a thorough leak test done to the system to find out where the freon is being lost.