If you want some real good information about working on your A/C I suggest 
you invest in an excellent publication that Haynes puts out that is interiorly 
focused on heating and air conditoning. Most of the the information is about 
air conditioning. It does an excellent job of explaining and illustrating the 
air conditioning process and also goes into elaborate detail regarding 
retrofiting from R-12 to R-134A. I've been asking a lot of questions about A/C
to a lot 
of local people in my area and find that they don't really understand it 
themselves in many cases. That has been proved to me by the fact that I've had
my 
92 Pontiac Bonneville A/C worked on the past two summers and neither 
professional shop could get it to work right. I decided that I could do as
well, or 
bad, as they did and decided to learn A/C myself. I've since switched my car
A/C 
over to 134A after replacing the very plugged expansion orfice and putting a 
new fluid line with a filter in it. I also decided to replace the evaporator si
nce the car has 170,000 on it but runs extremely well and gets 31 miles per 
gallon on the highway with the A/C on. The air now works very well. 
The other reason I decided to learn A/C was that I am at the final part of my 
project of changing my 87 gas LeSharo over to a GM 3800 and want to have the 
GM compressor and other A/C components married to my cab ceiling A/C. I'm 
just finishing that project and will check and charge the system next week as 
soon as the rest of my A/C equipment comes in.

Here are a few things you will want to consider as you approach your A/C 
project and/or have someone else do it:
-If the system has had a leak for any length of time you will want to change 
the fliter/drier/receiver assembley (it will likely have become saturated with 
moisture)
-Make sure all o-rings are replaced on any connections that are dissasembled. 
Be sure to use mineral oil to lube the o-rings before assembling them.
-When the system is reassembled make sure it is evacuated at least an hour 
and a half using a vacuum pump that can pull down a deep vacuum. Many people do 
it far less than that and don't remove all of the moisture that is in the 
system. You're correct about the problems that can and eventually will happen
if 
the moisture isn't removed.
-After the deep vac, isolate the system from the vacuum pump with the 
manifold gauges both closed and make sure it can hold a vacuum. If it doesn't
hold 
the vacuum you have a leak which you need to find and fix BEFORE charging the 
system with refrigerant.
-If you do decide to retrofit the system to 134A like I am doing to my 
LeSharo, make sure all the o-rings that you can replace are replaced with
neoprene 
ones that can stand up to the effects of 134A
-Make sure the system has enough system oil in it and that it is compatible 
with the refrigerant your choose to use. The original R-12 system uses mineral 
oil. If you retrofit to R-134A you will w need to to use Ester oil with the 
R-134A and remove as much of the old mineral oil as you can from the orginal 
components of the system. Leaving some of the mineral oil in the system is now 
okay even though at first many people thought it would mess up the retrofited 
system. Mineral oil does travel well with the R-134A and so the Ester oil needs 
to be added to the retrofited system as per directions by companies like 4 
Seasons, who also have a good , but limited, piece of literature about 
retrofitiing.
-If you choose to retrofit with 134A, be aware that you will only adding 
about "80 to 85% of the amount of R-12 the system originally had". Some people 
even advice only using 75% if the vehicle will operated in a climate that is 
often hot and humid.
-Also make sure you have a high pressure cut off switch in the high pressure 
side of the system if you decide to retrofit. I think the original LeSharo 
system had it but I am not sure.
-Refitting will also require changing the low and high side service ports 
with R-134A style ports that work on a R-134A capable manifold set of gauges.

I'm going to quit at this point and again encourage you to get a copy of the 
Haynes A/C and heating manual. It is only about $15 and covers many different 
types of A/C and gives a lot of specific information in tables at the end of 
the book. Everything I've shared above is included in it and a whole lot more.
Be careful if you decide to do it yourself. There is a lot of pressure in a 
system that has a charge in it. Opening the wrong valves can cause some real 
injuries to skin and/or eyes. Wear protectived glasses and gloves.
You CAN do it yourself if you take the time to do some reading and get access