Whenever you are pulling a trailer, the larger engine you have, the easier your trip will be. In addition, in many instances, a larger engine will consume less fuel than a smaller one working harder. Because the larger motor labours less, its reliability will usually exceed a smaller stressed unit. I have never been disappointed by having too much power, whether it be vehicles, tools, boats etc..

I have friends that have towed with small V-6 motors. It can be done, but not without problems. We pulled a 3H BP steel trailer with a 5.4 for a couple of years. It did the job pretty well, and we live in western PA with many hills.

The difference in pricing a used truck with the two different V-8 engines will be minimal, based solely upon their displacement differences. Look for the truck in the best mechanical condition first and then factor the engine size. If you can't find a truck with a towing package, the necessary items can be later added piecemeal. Unfortunately their total combined costs will exceed that of a used, factory installed package.

If you know of any mechanics, talk to some of them.  Before I bought my last truck I started talking to all the mechanics I knew.  I let them know what I was pulling, what I thought I wanted and asked what they recommended and what they would recommend to stay away from. 

It was helpful to me because they mentioned a lot of similar things.  They mentioned what problems they've seen in certain engines sizes and what years to stay away from so I was able to narrow down my search.  They didn't steer me wrong either, I've owned my truck for nine years with no major problems.