Partly depends on the dog, and how regularly they cleaned the van. Wouldn't put me off regardless as you can get rid of the smell with a good clean, some air fresheners and a little time. Especially as you mention a seat cover is fitted.
I'm sightly biased as we have a dog but equally I hate the idea of a vehicle or house smelling of dog!
We have a whippet so he has short hair and neither vehicle or house smell. We've got friends with bigger dogs (and/or longer haired dogs) and it's definitely noticeable.
If the van looks good with this aside, I'd go for it!