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Don't open the system unless you have to. If the charge leaked a little at a time you may be able to recharge. If the system has been dormant for a while the seals may have dried out, it's just hard to predict when you deal with an A/C system. It's kind of a do or die thing if you are going to recharge with 12 due to the high cost. I would replace the hoses and seals and the accumulator, if you stick with 12 you can leave the orifice tube alone. If you convert to 134 all the new seals sold by GMSPO are 12/134 compatable and any new GM supplied hoses are barrier type for compatability with 134. A 134 conversion CAN be made to work well but it has to be done right. If the car is a keeper I would seek out a good shop that specializes in A/C work or a dealer who has a guy who does nothing but A/C work and turn it over. There are a lot of opinions about what the correct way is but there is only ONE way to do it. If a conversion is done correctly you should be pleased with the operation, it ain't 12 but it will keep you comfortable. Yep, you need a set of guages. I have seen guy's use the Freeze 12 stuff with good results.

 

first thing i'd do is check for leaks, vacuum all the remaining gas out, fill the system with nitrogen, put a gage on it and mark what pressure the gage is at, then take some spray soap (i've found 409 works best) and sqirt some soap on all the o-ring and pretty much where ever a connection is made in the system, if you see bubbles theres your leak. but keep checking there maybe more then one, if you don't come up with anything leave the pressure in the sysem for a few days and see if it leaks down
Note* I would only put 150 - 200 psi of nitrogen in the system, to much pressure will break and blow out your o-rings.
As for using R12, there are a few good gases to use in its place like freeze 12 and Hot Shot, if you have a hard time finding these try finding a marine a/c shop as most marine a/c units still use R12. Hope this helps you.