The general consensus is as long as you are running a good tune on a healthy engine you're fine around 220-240hp before the stock rods let go. So, if by "turbo internals" you mean rods and pistons, you'll be fine for pretty much as much horsepower and $$ you want to throw at it. The cost balloons from there, soon you'll need a fuel pump, head studs, MAP sensor, injectors....yadda yadda yadda. Hit google up for D-series specific sites to do your research.