I remember reading once that Mercedes experimented with a V deisel engine for german fighter planes in WW2.
I worked with Mercedes V6,V10 and V12 engines in the 1980s and they were amazing for there power to weight ratio. They later got bought out by MTU.
Back on topic , if $$ isn't the main driver here have you considered a RR with a 4.4 deisel . I remember visiting Justin Cooper in tassie and if that's what hes chosen as his LR vehicle then it's a very good yard stick to measure from.
Bulletman

