No. Just..... no.
Towing a car trailer with a Ke70 is just asking for biblical amounts of trouble. Not to mention being very illegal.
You're saying that you want to tow a 900 kg sprinter, but the trailer will weigh at least 500kg. (anything up to 900kg for a hire trailer) I have a very light tandem, and it's 525kg. Thus you're looking at minimum 1400kg. Given that the braked towing capacity for a KE70 is 750kg, this is not going to happen.
Towing rule of thumb: The tow vehicle should be around the same weight as the item you're towing. Go and buy a cheap Falco-dore to tow with. They're cheap, fairly comfy to tow with, and you won't die a horrible death the first time you descend a mountain range.
Either that, or tell us when you're going to tow, and I'll stay home that day.