Even though the old R12 gas is obsolete they should be able to convert it to run the newer R134a, normally involves replacing all the seals, the expansion valve and the receiver/dryer, they add the lubricant to the system as they re-fill it.
If your heater slider has the ice symbols on it then you have factory aircon, a lot of dealers fitted the aftermarket kits and these normally have the switches in the area by the heated rear window switch etc.
I would say that if your car has been used with the system open to the air then most of it will be shot, the pipes have been disconnected and you may have got moisture into the unsealed system and this can cause problems with rusting. Your best bit is to go to an aftermarket place and get a quote as to what you might need as they normally stock parts that are cheaper than the original VAG supplied ones.
My '91 16V has factory aircon on it so if you need any help with part numbers etc give me a shout, I need to get mine recharged anyhow as its now not blowing out cold when the ambient temperature is high.
