Most vans do 20-30 K a year on lease by companies and drivers that couldn't give a flying fark. Thrashed, bashed and abused, then after 3 or 4 years tarted up for the s/h market by a dealer looking to maximise profit. and evan then they're usually more of a commodity than a personal item so it doesn't get any better, at 6 or 7 years old they're likely to start showing the scars of all the abuse and bodged bodywork repairs.
If I were after a s/h van I'd be inclined to check out the auctions, for one you'll be buying in a "real" state before the bodies been glossed over and any defects bodged to work as cheaply as possible. Sellotape>New parts, y'know. Pick one that looks clean and be prepared to pay a bit more than the dealers, but don't get into a bidding war against another private buyer, and have something set aside for cleaning it up and any faults that develop.
as for brands, there's really not as much between then nowadays as once. VW and merc are still considered the ones to have but watch for rust on late 90's onwards mercs. Transits have always been a reliable workhorse but evan stuff like renault masters that used to be junk aren't such bad products now.
If your budget allows try to go for more than the "bog standard" engine option, most vans will have 2 or 3 outputs for the same basic engine and given that you're towing this'll be usefull- the more common base models can only be described at best as "adequate" when moving themselves only. But these tend to fetch more in good nick and guaranteed you'll find jokers wanting well over the odds for a rusting tatty heap of junk just cos it's got the hi-power engine.