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If you just search through the posts here in this forum you will learn what goes wrong with the vans and how to fix them. If you want more specific advise you're going to need to give more info on what your going to do with the van. Just saying that your going to tow with it doesn't tell us anything.
Are you going to tow a jet ski or a 28 ft enclosed trailer with a race car?
What vintage or budget are you looking at, and do you want a cargo or passenger van? You most likely want an E250 or E350 for towing. Right now, there are off-lease 2013 E350's around for around $20k which I consider a SCREAMING good deal. This is in passenger van (Club Wagon) form. Cargo vans from these recent years might be even a few bucks less.
If you want to go older, I think the mod motors got new improved heads around 2005 or 2006 where they had a few more spark plug threads and were not quite as apt to spit plugs. So 2005+ is better than 1997-2004 in that regard. Vans never got the 3 valve 5.4 or 6.8, so there is NO issue with spark plugs sticking as is the case with 2004-2008 pickups and Expeditions.
Early 5.4 mod motors (1997-1999 maybe) tended to have a few seeping head gasket issues as I recall, and they got more power maybe 2000 or so with the PI (power improved) version.
You need more than a 4.6, but the Romeo 4.6 got power improved heads circa 2001 and from 2001 to 2004 maybe, there was an issue with the 4.6 head castings where the rear valves would carbon up due to bad cooling and eventually burn (I got a new pair of heads under warranty on my '02 E150). This was a 4.6 issue only.
I don't know of any trans issues especially on the heavier apps; my 4R70W has been fine. One of the trans guys should hopefully have any info regarding transmissions.
Good luck, and welcome to this forum; it is a great resource.
your budget is the biggest factor on what years to go for. if you're looking for a low cost van that'll do the job, get a 94-earlier diesel with the c6 tranny. these trannies are as near to bulletproof as anything you'll find. it won't be the fastest thing towing up mountains, but it'll do the job for many long years with a minimum of expense along the way. newer diesels are a lot more expensive both to buy and to repair when needed, but make more power - so thats a trade-off you get to decide on.
i'm recommending a diesel because they're significantly better on fuel and really are designed to be worked to their limit constantly
If you're looking for a V10, I'd look for a 2000+ to get the power boost vs what was available in 1997-1999. If you can afford the price, look for a V10 from 2005 or newer to get the 5 speed Torqshift transmission.