You can run stock internals if you want around 400-450whp. I ran 417whp/301tq for about 9 months, then i built my engine for more power never ran into any problems. As far as the transmission, that will vastly depend on how you drive. I had and still have a "mostly" stock SI transmission. and now im at 550whp/350tq. These engines are very stout, the tranny is the weak link. Launching at the track may prove some issues if you have 300tq but if you mainly drive on the streets you should be good for a very long time.

Make sure yout get motor mounts and lower control arm bushings (energy suspension makes a cheap set for us) This will prevent wheel hop. Wheel hop is the true killer of trans gears but a lot of power can do it too. It isn't a bad idea to keep a spare transmission around, or at the very least the gears you will probably break. If you go Supercharged 2nd gear is prone to breaking...3/4 if your turbo'ed.

If you have specific questions then fire away, but a lot of reliability question will be based on how you drive, how often the car will be driven at the track and things alone those lines...Simply put, the engine shouldn't have any issues at all completely stock if you stay under 450whp. People have had the k20z3 boosted for years and years without issues.