The first question to ask yourself is what "T" weight designation does your newly order van come with? If it is a T32 then you have just increased your pain and suffering, since you will need to ensure that any wheel you choose as a load rating of 860kg each, and your tyres will need to be a minimum of a 103 load rating. This narrows your choice of rim as well as choice and size of tyre. If you have gone T26/38/30, then you will generally fare a wee bit better in the level of choice out there for you.
There are so many threads relating to wheels and sizes etc, my recommendation would be take a look at some of them, and then pose additonal questions through those threads rather than starting another one. To help you make a start, because there are 48 pages of threads in the Wheels/ Tyres area, have a gander at these:-
Show your 18" & 19" wheels
Wheels ....which Style Of Alloys
20" Alloys What Et ??
Lets See Your Alloy Wheels
Wheel Identification Thread