gowesty do a 15" steel 7 " wide. why not use the standard 14"? are you getting your wheels from a supplier that will sell you wheels with the wrong offset and the wrong size tyres? worth checking before you buy. remember the supplier will tell you anything to get your money.
your 14" steels give you the correct speedo reading and the correct gearing for your van, if you but the right tyres for the alloys the circumrance will be the same as the og 14" so you can keep the og as a spare. remember to keep wheel nuts and bolts for the steels as these are different to the alloy ones.