Its all to do with the axle weight find these then divide by 2 so for example if
Your axle weight is 1500kg then the wheels need to be 750 kg or above
Dont forget you need van load rated tyres to match
Good luck
You will need someone with the same spec vehicle to check their vin sticker on the o/s under the bonnet, it will say the axle weight, then like kev says divide by two - per axle.
I would look for a reputable supplier who specialises in van alloys who will guarantee rating, do not forget if you get a 180 van they have up rated breakes so not all alloys will fit ( do not know if it is an issue with the size you are looking at but for standatd 17inch it can be a issue)
google race the road I have used in the past seem to know what they are talking about