Regardless of what you do, IF you convert it to R134a, you must replace all of the o rings. The R12 o rings are not compatible with the R134a and it will be a short time before the system is leaking from every joint.
That in itself is reason enough "to me" to think about replacing major components, because you will have all of the connections loose.
Air conditioning is not "cheap" on anything. House, car, whatever. To me though, you have the whole system intact. It would certainly be nice to have it up and running, regardless of whether you convert it, or are able to find some R12.
