The 5.9 Cummins is an awesome motor. May be one of the best. I have a 6.7, but have had several 5.9s. I think they're slightly better simply because they never had all the emissions bullshit. No Egr, no particulate filter, and only one catalytic converter. They get good fuel mileage too with a few mods.
What do you plan to do with it, dd, heavy towing, etc? What are you looking for, an auto or manual trans?
If you run stacks, there is an 8" minimum size requirement in order to be cool.