The brakes and searing are much better on the 3rd gen and I think your are right to go with no older that that. There are some good buys on the early 4th gens but you have the issues of the emission systems. If you use it mostly for towing there wont be as many problems. The people that have emission problems is short drives and no loads. I personally would look at new ones first to get a baseline. I don't know what's important to you, a Laramie or a base truck. You might be able to find a new tradesman for about the same price as a used Laramie. If you want an auto trans the 4th gen with the 6 speed auto is much better that the older 3 speed with an overdrive, witch you might have locked out most of the time when towing. Pre 2013 will not have Diesel Exhaust Fluid (DEF). The DEF systems are better, engine internals stay cleaner and more power and better MPG. If its possible to remove the emissions on a 2010-2012 in your area you can get the benefits of the cleaner engine and more power and better MPG and no DEF. What I have found is clean low mile 5.9 (pre 2007.5) are more money that the equivalent 6.7. Just seen a 2006 auto with 17,000 mi asking $37,500. I had a 1997, a 2005, both manual trans. My 2011 (emission deleted) is auto is by far the best of them all. Some will say you can not get emission delete tuners any more, they are available and easy to get, no one is advertising them as such, so you need to know where to look.
DRW or SRW? I towed my 40' toy hauler with my 2005 SRW and had no problems, I did need to go with a bigger tire to keep from overloading them, and I added air bags in the rear. But really a DRW would be a better choice for that large of trailer. My 2011 is a DRW and I will probably stay with a dually now, but I don't use it as a daily driver.