Smitty
Your question about reliability....I think you need to look at what breaks on these trucks vs. the power level...then ask yourself what reliability level you're happy with.
Like these guys are saying...the trans is a buy now or buy later deal. You can plan on running the one you have on smaller tunes...but just know in the future, you'll likely need to bite the bullet.
The factory fuel system is a POS. It leaks, doesn't flow/filter fuel very well. An aftermarket fuel system from Strictly, Irate, or Diesel Innovations can IMPROVE reliability on your truck. If you decide to do a DIY fuel system, make sure you take the time to engineer it as good as the systems available from the vendors.
The factory HPOP system will generally starting tapping out on injectors with a 100% nozzle...on the longer Pulse Width tunes...your IPR Duty Cycle will start to get to the unhealthy point....so you'll need a "medium oil" HPOP in there to keep that under control. Nothing is more reliable than a stock off the shelf OEM HPOP. Ask injector builders/tuners you're contemplating doing business with, what's the largest nozzle a stock 17 deg hpop can support.
The factory headgaskets start to get skittish at our above the 450 hp level without headstuds. Some will say lower, some might say higher. But my opinion is you'd need headstuds over 450 hp.
Valve/piston contact is a concern at RPMs over 3000, and if you start encountering drive pressures (think of it as boost pressure on the exhaust side driving the turbine side of your turbo) over 40 psi. The fix for this is pushrods/valve springs.
Finally...rods...blocks...etc...these get skittish on anything over 550 hp. Some will quote a handful of 650hp forged rod trucks...but tuning, use, owner history....there's a large variance here...so I'd say for any power level north of 550 hp, and to be dead set confident...you'd need to plan on a built up short block. $$$$
If you knew, that you're never gonna have a built short block, and that if this motor had an internal failure, you'd just replace it with a used running forged rod long block....than I'd set my goal at 500-550 hp.
But you don't need to get there in one step either....
I'd do the fuel system, HPOP, and turbo as time/budget permitted. Learn your truck, and continue to research injectors/builders.
Then I'd do springs, studs, valve springs, pushrods and the injectors you decided on.
Talk with as many vendors and customers as you can. The customer experience you have pre-purchase, will be representative of the one you have post-purchase.
Hope this helps. Remember....keep it fun!:fordoval: