When I was considering going 911 Turbo I did a *fair* bit of research.
My understanding is that the X50 package gives you the GT2 Turbos/ Intercoolers and I *think* upgraded transmission/clutch to handle the power. Easier to reach certain goals as with just tuning, you are where you would need turbo's to reach with non X50 obviously. Another option is the 996 Turbo "S" which I believe gets all the X50 stuff as well.
500-550 @ the tire and more torque should be doable on the stock X50 (k16) Turbo's. (Might want to research clutch durability though).
The 996 Turbo and 997.1 Turbo engine are still the legendary metzger engine which disappeared with the 997.2 Turbo. Seems to be a pretty huge price gap and the 996 will be much easier on the budget.
I'll let a true Porsche guru chime in, but I'd get either an X50 or Turbo S or a car that has already been modified with larger turbo's (like that white one you posted before).