Am I correct that you mean cam belt where you refer to drive belt?
If so, my local garage says it's the same engine as the Camry. They now have 5 Alphards on their books, so they 'know their stuff'. If you are anywhere near Derby, I would highly recommend them - CK Automotive.
Thanks for the info about the badge under the bonnet - will have a look later. Have you tried an online translation tool?
As for fuel economy, following a full service, our 3.0 litre managed 30mpg on a long run recently - I suggest that isn't bad for such a big car. And there seems to be no issue with Low Emission Zones (I haven't checked London, but the others are all fine). So I don't feel any need to go back to a 'dirty diesel'!