Just an update on my research, coming back to the 2ar-fxe Toyota 2.5 na from the rav4. Spoke to a guru about these engines and he said not to worry about the auxiliaries as I can just fit a alternator and water pump from a normal 2ar-fe engine. So that's still an option. The guru said I'd have to mount my current A6 gearbox to it as the Toyota gearbox is transverse.
Not ruling out a V8 either, as I've found a narrow body A6 like mine fitted with the 4.2 Audi V8. I'd be looking at the rover V8 3.5 or a lexus v8. I've ruled out the Toyota Hilux diesel (2LT) as they're not as reliable as I thought.