Re: E350 van in gas or diesel for towing?
I would say that the diesel will be the best choice hands down, but I would be inclined to wait a while and see how this new diesel will pan out. I had a 1999 with the older 7.3L Power Stroke and I regret every getting rid of it.
A van is fine for towing, the Ford vans use a solid chassis that is very similar to the pickups. I did have to do ball joints in mine at about 70K, but the replacements went over 140k. The trans lasted 187K and wasn't as bad as I thought it would be when I finally rebuilt it.
It towed like a champ and could handle anything I hung on the rear. I regularly towed a 28' eclosed tri axle trailer which weighed in at just over 9500lbs.
I have an older van that I use now to tow my boat that weighs in at about 4800lbs, that's only an E150 conversion with the 5.0L but it does fine for the short distance I have to tow these days. I rarely go more than an hour to a launch site.