I replaced the AC Compressor on my 81 300TD this spring. Had a bearing making noise so I got to mine before it seized. YES, get the NEW R4. My parts person doesn't even sell rebuilts - said he had too many come-backs. Got mine for about $240.00. I converted to R134a at the same time (I had done my 71 250C a year earlier). Replaced the receiver/dryer AND the expansion valve ($17.00). The expansion valve is R134 rated.
Other than being a little noisy at high temps (above 100 or so) all is well. I am satisfied with the conversion. Since I have the wagon (and it's black) it takes a little while to cool things off but not too much longer than when I had the original with R12.
Just my .02.
Dan Taylor/ Tulsa, OK MBCA
'84 300D/'90 Jaguar XJ6/XJ40