Your guess is probably right on.
I wish that there was a cheap way to fix your air (I'm into "cheap" fixes myself, brother). But, if you can find a can or two of R12, it'll be expensive. And, you very most likely have a leak in the system, so it'll just be a temporary fix; plus, leaking R12 into the atmosphere ain't good. Honestly, saving up to do a correct R134a retrofit is the best thing you can do. Sorry.
So, I'll try to help you out, with your other questions:
Your low pressure switch is on the drier / accumulator. It can be temporarily jumpered, FOR TESTING ONLY.
A vacuum pump is connected to the system with a standard AC gauge set.
There are "sniffer" leak detectors, and there's dye that can be put into the system. Another method, is if the system will hold a vacuum, after it's evacuated (with the pump). However, the best method is the next point.
One added benefit of a correct conversion, is that all of the system's o-rings should be replaced. Thus a lot of times, the conversion will also correct the fault in a system that has leaked out.
The sticky post at the top of this forum is a good writeup by MAKG on doing the conversion. Basically, and simplified, it involves replacing the drier, replacing all of the o-rings, flushing the system's components, adding the correct amount of compatible oil and R134a refrigerant. Once you open the system, evaluate it for "black death" (trashed compressor) and if the orifice will be reusable (it's really better to just replace it).
Hope this helps some, and good luck.