You can get whatever alloys you like. If you are loading up your van then you might want to get suitably rated ones. Mine are from an ML which has suffcicient capacity. Just choose the right tyres to suit. I have 16" alloys shod with 225/65/16R tyres. Obvoiusly 17inchers will be 225/55R or something.
Definitney lower the van. Full height vans with alloys on don't (IMHO) look good.
Definitney lower the van. Full height vans with alloys on don't (IMHO) look good.