EnyChevyUK

I know the factory wheels are 15'. At the moment they are on the van, looking a bit tatty with the 235 winter tyres.
I also have a set of 255/70/15 tyres. I am thinking of buying a new set of wheels for the 255's but I am unsure of the different specs of 15' wheels.
What does 'back space' mean ?
As my van has 'running boards' the factory wheels are narrower than the width of the van and I want a wider look but don't know what spec is best. Do I need a bigger 16' wheel or a wider/deeper 15' ?
Also, is the van classed as a 'commercial vehicle' when buying wheels due to its weight/size or are all wheels suitable ?
I have seen a set for sale with the following description :
CHEVY 5 STUD
4.75 PCD STUD PATTERN
FRONT 7"x15
REAR 8"x15
BACK SPACE 4"
Is this suitable for the Express ?
I had a G20 a few years ago and that had a different size on the front than on the rear.
Thanks for any advice on this. How is your van set up ?