I had this fear when buying mine had high k's (though i always thought 300,000 was high lol , it had (184,000ks stock as rock) But its not the age but more the way it has been treated i was lucky to get mine off a old couple which were the original owners since being imported hence it being a (4 door r33 ) found they were less molested in most cases.
But since then ive modded the 1st stage bolt ons being the FMIC,Turbo back exhaust,apexi panel,boost controller etc, Did the major service timing belt, harmonic balancer, basically looked after it with regular service etc and hasn't missed a beat..
I mean i use mine as a daily but like what has been said if ur not planning on using it a track car etc i wouldn't bother going and rebuilding or replacing a working engine, mine is currently at 274,000ks with stock turbo, and besides the mods all ive ever replaced has been the timing belt and balancer and belts ... basic major service ..
If your really worried about the life of the engine just get a compression test