A/C system

Hi, we have a the vehicle listed above that we bought on craigslist. Everything worked great when we got it, including the air conditioning. About a week after we got the van, the A/C stopped working. We took it to a mechanic who said there was an A/C leak and the guy who sold it to us must have just put in a can of R134A right before he sold it to us. The mechanic said there was a leak in an A/C line going to the condenser, which we had replaced. After that, the mechanic said the compressor was bad? I don't know how it could have worked perfectly (including the compressor) a few days earlier and then have the compressor be bad? My wife and I are seniors on a very fixed income and we can't afford to have a mechanic fix it so the car has gone without A/C for over a year. However, our summers get over 100 degrees each year and we need to get it fixed. My son is pretty handy and said he would come over and change out the compressor if we buy the parts. We were thinking of doing that until one of our neighbors said that if an A/C system hasn't been used in over six months, that it is best to change out everything. Is this true? I thought we could just replace the compressor, vacuum out the system, replace the R134A, and have out A/C working again. What is the truth about what needs to be done?