If locked on a diesel I'd opt for an older Cummins without the complexity (and expense) of the latest and greatest.
Frankly, any diesels built/sold in North America in the last 10-15 years is going to have complexity. They haven't been making true mechanical diesels since the late 90's or so. The 2007 and prior diesels do avoid a lot of the emissions complexity, but they're also sitting in 10+ year old trucks and they still have complexity in other areas (fuel delivery). By many accounts, the newer diesels aren't all bad...they've actually seen significant improvement over the last few years. I would say that the 2007 (when a lot of the new emissions were put into place) to 2010 seems to be the more problematic years for diesel emissions. 2011 and beyond has seen a lot of the bugs worked out.
The issue with diesels off road is the weight. That takes its toll on the truck, and the trail.
I don't think that is necessarily true. Firstly, it's a given that you'll have to modify or tune a diesel truck's suspension for offroad/overlanding driving. Those trucks are set up to carry a lot of weight, so unless you are planning on carrying a huge camper, the suspension is going to ride too stiff. Secondly, it's a 4x4 truck, and if you drive it as such (as slow as possible, as fast as necessary) offroad and not a trophy race truck or buggy, I think 3/4 ton trucks, even diesels, will hold up just fine. The 3/4 ton's aren't all that much bigger than full size pickup's, and if there is an area where a 3/4 ton's weight will be an issue (deep mud), chances are a 1/2 ton or even midsized 4x4 isn't going to fare much better.
OP, if I were in your shoes and towing 8,500lbs on a regular basis, I'd definitely look at diesel options. I know there are 1/2 ton trucks that are well capable of handling such loads, but in terms of fuel economy and engine work, the diesel is going to fare better IMO.
For earlier diesels, I'd look at 2005-2007 era trucks; they avoided a lot of the emissions complexity, though some did have EGR by that point, and are generally pretty reliable. Downsides would be that the truck itself, and the potentially the engine, are pretty old and likely well used at this point. Examples: 2005-2007.5 Ram 2500 with 5.9L Cummins; 2006-2007 Chevy 2500 with 6.6 LBZ Duramax. You could go with older versions of both engines, I just think you're going to have a hard time finding a vehicle that is still in good condition (but maybe that's an option if you intend to do a restoration or vehicle build).
For more recent diesels, I'd look for 2011 and newer trucks. All of diesels by this point have the full gamut of emissions (EGR, DPF, DEF), but you generally see a lot less reliability issues with these newer trucks than what you see with the 2007-2010 trucks. These trucks are a lot newer and in better shape, even used examples, and the interiors and general fit-and-finish saw a lot of improvement in these years across all brands. Downside is they're going to cost more. Examples: 2013+ Ram 6.7l Cummins in 2500/3500; 2011+ Ford F250/F350 with 6.7L Powerstroke; 2011+ Chevy 2500 HD with 6.6l Duramax LML.
*I'd avoid the older Ford Powerstrokes (some of them were really made by Navistar International) like the plague. The 6.0 Powerstroke (2003-2007) and the 6.4 Powerstroke (2007-2010) had a lot of issues, and the fact that Ford used both engines for only a short period of time should tell you a lot about their longterm reliability. The 7.3l Powerstroke (really a rebadged International engine) preceded both engines and was very reliable, if a little down on power, which is why people are willing to pay so much for it on the used market.
