Yes, it does sound like you have lost refrigerant. It is totally legal, and usually the accepted method to recharge with r12. If you have some yourself, it is even legal for you to charge the system yourself. What is NOT legal is for a non certifed individual to work on someone elses car when R12 is involved.
R12 is EXTREMELY expensive these days. For this reason, you need to ABSOLUTELY ENSURE that there is no leak when you recharge the system. If it has gotten low, it stands to reason that there is a leak somewhere. You need to find a reputable shop, and let them look for a leak, repair it and recharge.
One of the alternatives is to convert the system to R134. My personal experience in changing MB's to R134 has not been good. I have converted American cars with no trouble, but the MB's have eluded my success.