I've got to agree with Larry on the new R4, even though I'm currently "rebuilding" a spare one on my bench right now (more for my education than anything else). The original Harrison R4 has gone thru MANY incarnations with MANY manufacturers making genuine R4's over the years (SAE threads, metric, stepped ports, "O" ring ports, shaft nut, no shaft nut, etc. etc.) A new R4 is cheap enough that when all is said and done, for a "keeper" vehicle, it's not that big a deal for the peace of mind.
That said: a couple of listers here, and on the mbz.org diesel mailing list, have used pre-owned ones from the wrecker"s yard. I think I'd only do that on a real utility-only vehicle.
Most common failure is the same as the A6 series (HR, HD, DA, etc.)....front shaft seal.
Whatever you do, DON"T ever use a chain store rebuilt R4 (Murray, Evercool, Four Seizens). It will DIE.
The Golden Rule
1984 300SD (bought new, sold it in 1988, bought it back 13 yrs. later)