I know it is a mine field just looking at buying our first motorhome.My lawn tractor has a 480cc V twin petrol engine and is capable of towing a 2.25t landrover on evel ground.
Doesnt really answer your question but a 2.5ltr engine is capable of towing a lot more than 1500kg.
As already said, the more important issue is your vans train weight (maximum combined weight of van and loaded trailer)
the trailers fully loaded weight is applicable....even if towed empty.....so a 1500kg trailer may only have a true payload of around 1100kg/1200kg
If your actual load is 1500kg you will need a trailer with a gross weight of around 1800kg/2000kg