I bought the '06 Cummins 4wd quad cab w. 42K miles for a damn good price about 10 years ago. The 5.9 runs perfect...only problems under the hood have been the AC compressor locking up and ruining the serpentine belt (fixed)...and coolant leaking at the fitting on the back of the block (will be fixed). Oil and fuel filter change every 7-8K
The truck's biggest issue has been the dreaded 'death wobble', which would only go away with new tires. I plan on replacing the older steering linkage with the newer heavy duty parts. Have everything needed...waiting for warm weather.
Drivetrain...warranty covered a bad bearing in the transfer case, and the transmission tailshaft cracked because of a u-joint failure. Had that replaced. I put in a new driveshaft for insurance.
The fuel system has been problem-free so far, but I do want to put a lift pump outside of the tank ..any advice/suggestions on which brands?? I was lucky to find a spare Reman. MoPar- Bosch pump for cheap at a swap meet. Good to have a spare.
As far as body...Illinois sucks, period...and the weather has been hard on the sheet metal. I'm on the hunt for a rust free bed and a rear pass. door (anyone have those in silver to sell??)

Other plans include new exhaust, and gooseneck hitch. Maybe aftermaket heavy duty bumpers

plan to do the motor when the ring breaks on #1 or #6 fass makes a good stand alone system you might want to try.

I though the in tank pumps in 2006 were good?
If you are considering an external pump, I agree Fass is good. Look at the Fass Titanium where the pump comes with the good fuel filters and water separator as well. I installed this after replacing my injectors so the new ones will stay clean.

If it were mine I’d leave the in tank pump.... if age and miles on it are high I might take it out and clean the inlet screen. Op already indicated living in the rust belt. Last thing I’d do is to splice the line and have a pump hanging on the frame that’s just going to end up corroded and leaking.

“If” you do go to an external pump you’ll have to drop the tank and make a new fuel inlet for supply.