Fuel transfer pump and filtration, FASS etc.
Gauges for fuel delivery pressure, exhaust gas temp, boost
turbocharger such as BD Killer B
replace turbo discharge hose with Mr. Bob's Distributing hose
low restriction muffler, check backpressure across catalytic converter (if 2004.5, 325 HP)
You say it's high mile. First thing I would do is have the injectors checked. They tend to have flow balance issues by 150k miles. Injector flow balance is critical as it can be the difference between a running motor and melted pistons.
x10 on the injector thing. If you see black or white smoke under light loads or at idle, then you may need new ones, which is at least a few grand, which sucks.
But.....if they start leaking more fuel than they're supposed to after injection events, it can create a hot spot on the piston, melt it, and score the cylinder wall. Hello complete engine rebuild, which runs about 7-10K.
If the injectors are ok, then you're golden. The Cummins are very good engines other than that.
As far as power or mods, how much money do you have to spend? You can basically bolt on your way to 1000rwhp with enough cash and a beefy transmission.