Ok, most 6 liter Pstrokes didn't have problems. Allot of people would beg to differ. Sort of the same for the 6.4.
You want reliability and not a gear head? This is not the place to start.
Look for the best HD gasser truck, new enough to have a 6 speed trans (unless you find a cherry old 8.1/Alli GM) with the lowest miles your budget can withstand. Any flavor.
If you can do 2wd, you can actually find some deals compared to the majority and more popular 4wd trucks.
$15k will get you ALOT more truck in an ext cab 2wd vs cc 4wd that most people want.