These trucks are sold with both automatic and manual transmissions. The engines are considered small for trucks, with the base engine being the 2.8L LK5 I4. A much more powerful 3.5L L52 I5 engine is sold on the Z71 package and is an option on other models. Even that engine is underpowered compared to larger truck engines.
The number of variations on these trucks seems almost endless. The letter/numeral naming system for these variations can be even more confusing. The ZQ8 edition, for example, features a lower suspension and is more "road-tuned" for a sportier drive on pavement. It also comes with 17 inch wheels, and should probably not be taken off-road too often. There is also an XTreme edition of the Z85 package that comes with fender flares and side skirts, along with 18 inch wheels. I hope that didn't confuse anyone.
These trucks are all fairly new, and therefore not well represented in the used truck market. You can probably find some off-lease at dealer lots, but you'll always pay a huge premium on used cars bought at a dealership. I would suggest looking on eBay, where there are often dealerships selling off some of their off-lease vehicles for a big discount just to open up more space on their lots. They know that they can unload a vehicle quicker on eBay, so it offsets the discount they have to sell the truck for. Some even sell unsold new vehicles from the previous model year on the popular auction site.