As usual, check with a local workshop...but as far as my limited knowledge goes :
The factor of an engines life is depenant on a) How well you treat it, servicing etc etc. B) What mods are done to the engine, the more boost the less life etc etc. c) Luck.
The SR20 has either the T25 or T28, pretty sure its a T28, which has a steel wheel and is pretty useful up to about 1 Bar or 15psi (yes its really 14.7psi!), the turbo could probably do upto 18psi but you start running into heatsoke intercooler problems and maxing out your injectors. Of course other turbos are available.
SR20's can be prone to blowing headgaskets and know to lift their head under heavy boost (this is a matter of opinon.) Which is solved by having the head bolts drilled out and replaced with larger items. Or just add a metal headgasket, and be careful.
Intercooler, many people can help you out in Sydney. Alot of people use the HKS/Apexi/Trust etc etc...personally I'd save some cash and use someone local with a good rep...ask around your area. Any good intercooler shop should be able to get your desired result on the piping side of things.
Computers. Apexi PFC, you really can't go past it for a street appilcation.
If you after big power, one of the first things I'd do would be upgrade of the fuel system. The SR20 is a great motor, the boost hit is very powerful, and they produce results from their mods.
What car is it in ? 200SX, Silvia, 180SX or GTi-R ???