>>28587762
I would argue the Getrag 420G is at least plentiful enough from all the fucked E39 540i's, to have 1-2 spares for $2-3k each, if you're wanting long-term ownership. Direct enough swap, if your first 420G breaks, and you can reuse parts.
We could go balls to wall, spend $5k for a Nissan CD009 trans swap. Or go Full Retard on a $10k bulletproof T56 Magnum trans swap. PMC Motorsports, and other companies make bellhousing adapter plates.
But we're talking about attainable dream cars, here. Can have E38 740il easily for less than $10k, for a really nice one.