The 07-09 are prime candidates for deletes as the emissions systems are less than adequate, mileage is pretty bad, emissions systems are aged way beyond their expected lifetime. No AISIN in them, the 68RFE is likely to need attention at that age unless it has been done. HG problems with programming can be a problem.
The 10-12's have better emissions programming, less issues with HG but deletes can cause problems, better NVH and more options if you are looking for that.
Steering was not fixed until 2013+ trucks and you will have to check as it looks like some 2013 trucks did NOT get cross over steering. 2013+ you will find more AISIN units but unfortunate more 3.43 gearing.
The 4th gens had more of the crew cab if you are looking for extra room in the rear as opposed to the mega cab. Same wheelbase as the quad cab with an 8 foot box. 2013 + will be DEF trucks, less EGR problems, aq little better mileage, but, the SCR system are basically junk and they have a tendency to kill the DPF and catalyst when they go bad.
Later 68RFE's have some of the needed updates but they almost all need some pieces or a rebuild to current best practices and apply trans tuning for best results.
The HE351VE turbos on a non-SCR truck are more prone to issues, a lot can depend on how they were used and any updates applied. Controllers failing are the most common but getting carboned up and causing the failure are also prevalent. When it fails no power or really high drive pressures. All of that trickles down to the HG issues so it is not uncommon to see am HG problem with a failed turbo.
Injectors can be a big deal just like the 5.9's, almost guaranteed if an injector fails it will take out an HG. Just like the 5.9, you have to factor in a set of new injectors for reliability.
Mixed bag when you buy these used trucks and do not go over them in minute detail plus use them as you intend to see what happens.