Hello Holly,
Welcome to the Forum
Welcome to Alphard/Vellfire ownership.
Gas mileage is a function of the distances you expect to cover.
A gentle right foot goes a long way, and the V6 engines are smooth and powerful.
Your will have a 6 speed gearbox, which helps.
There is a thread on here, a Megathread regarding gas mileage, actuals.
I believe the headunits on your model, at least the OEM ones are able to be changed to English.
Alpine is a good brand, I would not rush to change it. Take a photo of the Alpine when it arrives.
You do not have to change the speedo to MPH but most do.
It's not a difficult job. You need to buy and fit a different speedo chip.
The dials, or meters as the Japanese call them, need to be removed, the chip changed and the dial assembly replaced.
Lots of this is covered herein.
Lots of reading to do while you wait for the boat to arrive.
You will need an MOT before you can register the van with the DVLA.