Is there any R12 left in the system, if so it will need to be evacuated to do a conversion. Take it to a shop for this.
Below is the things a shop could do or you could do once the R-12 has been removed.
2st thing you should do is a vacuum test to see if it will hold a vacuum, (or a pressure test and see if it will hold pressurized nitrogen gas). No reason to start adding Freon if you have a leak. If its been out of commission for a long time and if its not under pressure, then there is a good chance that corrosion has setup and you will have to replace most of the parts anyway.
So get a good vacuum pump or nitrogen and test to see if there are any leaks.
See if you can turn the compressor by hand (not the pulley but the inside part of the clutch).
If all that test out ok, then you will need to at a min flush the system and replace the accum/drier (which ever it uses).
Barrier hoses should be installed to replace the old rubber hoses if you intend to use R134a
The compressor oil should be dumped and replaced, most places recommend ester oil on conversions vs PAG that is normal for use with 134a, this is due to ester being ok to mix with any remaining mineral oils left in the system.
You may as well get a new Parallel flow condenser as well, a generic that you can it the space of the old one. The condenser takes a lot of abuse and is a likely source for leaks.
for a shop to do this I would think 1-1.5k easy. if you do the legal parts your self 150 for a cond, 150-200 for a compressor,75 for hoses, 30 for flush.
You may find a shop that still works on R-12, or do a simple R134a conversion (fittings, flush, drier, evac, fill) but realize you are dealing with old parts that likely will fail under pressure, so if it works it may be short lived.
Welcome to the fun world of AC.