The a/c was dead in all likelihood because it leaked. In any event, it had R12 at last operation and you will have to convert. Get a start on it by drawing a vacuum to determine how big a leak you have, then partly charging with R134 and finding the leak. From there, you can estimate how much working a/c will cost. The compressor may be OK or not, depending on how much oil remained in it and how much moisture was in the system. You will at a minimum have to replace the accumulator, the R134 charge, and the system oil.