If you have the money and do not mind the gas mileage, then get the Alphard V6. Being included in the JPEPA discount, it is more bang for your buck as compared to the 2.4L's. And then again, the 2.4L's give only slightly better FC but with a big difference in the performance department.

I'd leave the Previa alone as it is definitely too small for your purposes. I doubt you will be able to put much else besides the wheelchairs if all the seats are up.