Was about to put something similar .
Anyway go use the search button and read peoples build it will give you a realistic idea of the cost ,a "simple RB26" swap is a 10k$ swap and you dont have any tune or any idea of how the engine will reatc at 1st start up . I did extensive research on the cost of diffrent swap versus power and reliablity and the diffrence between an RB25 and 26 in money overall is just arond 2500 $ but the 26 can be pushed further . I based my calculation on a 500 hp build . I didnt wanted to hear about how unstreetable it would be but it was a personnal mark i wanted to have just to be able to say i got 500hp " POTENTIAL" and maybe do a dyno run too proove it then bring it down to a more streetable tune . Anyway i added everthing i could possibly imagine including bolts and oils and what not to do the work myself and the numbers are substential .
If you want to only and truly do just 300 hp reliably id go with a RB25 without a doubt . Sounds great looks greaty it keeps the car somewhat unique and it still has a I6 . I got nothing against SR`s they can make easy 300 hp but the torque is not as much to the rendez-vous then on a RB25 and for a street car torque is what put the fun in your ride ,its some what fuel efficient compare to the V8 .
In the end its all about if you really want to only have 300hp because their is very few people who can truly say i have enough with my 300hp i dont want to go further most people want more .
And if your wondering Im going for a VQ35 Turbo .
My 2 cents