The Cat D register is the problem and will affect the vans resale value even with low mileage.
If you don't know what the reason was for the van being written off then you aren't going to instill any confidence in the person considering buying it.
You could get a full AA inspection to help a potential buyer.
Honestly, I wouldn't touch a cat d vehicle. I've seen cheap replacement panels used in the repair rust out in a few years. So whilst it may look good today it doesn't mean it will always be like that. Its all about risk and I'd rather not take that risk. In the trade we wouldn't consider trading a cat d vehicle in tbh, so my prejudice mainly derives from then.
Another thing that would put me off is the replacement engine. I see I'm not the only one to question this.
http://www.whatcar.com/news/cat-d-need-know/.