GAP insurance is an alternative approach, but be aware that there are various types of GAP insurance, each covering different things.
Return to Invoice will pay the shortfall between the insurer pay-out and the invoiced cost of the van. You will need an official purchase invoice for this type of cover, so may be problematic when buying from a private seller. The amount paid out may not be adequate to replace the van like-for-like if you negotiated a discount at the time of sale. and it may also be problematic if you've added value to the van since purchase (i.e. mods and upgrades) - you'll need to read the policy conditions carefully to determine if added items are/can be covered.
Finance will pay the shortfall between the insurer pay-out and any outstanding finance amount, so obviously only of use if you used finance to purchase the van. Also worth noting that, as your outstanding finance amount reduces, so will any GAP payment.
Return to MV will pay the shortfall between the insurer pay-out and the market value of the van as at the point of purchase. If the market value of the van has increased since purchase, this amount may not be adequate to replace the van like-for-like.
Vehicle Replacement will pay the shortfall between the insurer pay-out and the amount it costs to replace the van new like-for-like. This protects you from the rising cost of new vehicles, but the GAP premium tends to reflect this. Also, purchasing a new van with the same mods/upgrades already applied may be problematic (and re-modding and re-upgrading may not be covered).
Others. There may other, niche GAP products that provide a variation on the above. I can't think of any, but I've been out of the insurance game for almost 5 years, so there may be new products on the market now.
So, you pays your money and you takes your choice. My preference is the agreed value route and to ensure that that value is reviewed and revised at each renewal. The aim is to try and ensure I would end up with a like-for-like van without being out of pocket, should the worst happen. That's not to say agreed-value policies are always straightforward - estimating the value is often tricky and so too can be getting the insurer to agree the value you want!