As an ex mechanic my idea has always been focus on the condition of the van and running gear,I've seen low mileage vans absolutely knackered and high mileage vans carefully looked after!
It's all about maintainence and care,at the end of the day they will be cheaper too because of the mileage so if there's anything suspect factor it into the price but I'd never be put off high mileage if the vehicle was mint, you could say components are more likely to go higher mileage but at the end of the day anything car fail whether high or low mileage.
People get put off old age vehicles too, I wasn't keen on my van but it was cheap clean and I know it was one of the best Diesel engines peugeot ever made so bought it and it's never gone wrong and I've never spent a bean on it apart from cambelt and service.
Just look for the best most looked after van you can get and try and see what history it's got and make sure you get it cheaper if it's got moon mileage😀