well if you want power definitely go for the r34 gtt
as the rb25det is a very good engine from factory and i have seen some get around the 300 kw mark with a couple of simple mods and wont cost a fortune
(however i am new to this as well i am aiming to get around 300 kw on my 34 as well)
the silvia as you said is a nice slick looking car but in all honesty that car would take a mint to get up to 300 kw purely based on the fact that
you would need to upgrade the internals, gearbox, clutch etc ( not saying that the 34 wouldnt need these but its just lighter on ur back pocket and more reliable )
hope that helped and let me know what you end up doing