You know what, maybe the R380 isn't that bad. All the ones I have driven were earlier ones and they were all slow shifting, in some case grinding, and weird feeling. I have not driven a later suffix one as fitted to newer trucks. Keith might be your best bet for a real opinion as he has the exact combo in his Disco and he beats on it. I have always disliked them so I am biased.
My point is that the ZF in your rig has been good until this point. An R380 will probably not give up the ghost in one shot, but you will probably be looking at rebuild time after 200kms of tough love anyways, about the same as a ZF. Then again I pulled a blown up R380 out of a Disco at 78K kms.
Trying to engineer all the failure points that might leave you stranded out of a DII is impossible, even a 300TDI can pretty easily fail, timing belt, relatively lightish injection pump, etc and a custom job by definition further compounds the issue.