going the 6 route bang for buck is rb30et ( get em complete with gbox for under 1k parts are worth nothing and will cop alot of boost like 18-20psi for a decent amount of time) rb30's parts are worth nothing to the point if you pop the motor just buy a new bare block they litterally are like 200-300 bucks, plenty of torque but sound like ass
( best cheap setup ive done, highmount mani, to4e bush, bigger pump and wolf 3d made 250kw on 18psi but was running out of fuel (with injectors and pump will crack 300 at say 20psi) and stock would be more than enough to have plenty of fun in the c210
more reliable but bit down on power is def the rb20det, with a rebuild these engines live to be flogged as most budget drifters can vouch and even without will take a hiding before only doing a headgasket usually. These will cost about 2k with a box but will last a bit longer if you restrict the boost to about 15psi, (best cheap setup ive built a 20 on is, cams, cam gears, injectors, 3071r, fmic and powerfc made an easy 260kw on 16psi with a very saweet power curve)
rb25det, this is a good im if your not gonna spend any money on the engine after first setup, will cost a bit to buy straight up but is a good reliability vs power setup, looking at around 4k for a complete setup (so the two setups above for the same cost as this std engine)
rb26dett, the most expensive to buy and to convert into any non gtr car, starting from at least 7k with a box, there is ALOT of modification, great power and the big daddy of the conversions you will pay for every part but it is worthwile
or my fav which i was going to build in my s13 before i sold it the rb25/30 a very underestimated for cost and majorly powerful (if you got the money 26/30)
with some bolt on mods such as plenum and cams and a forged bottom end i was looking into the 400kw mark
my build cost was only 5k doing the large majority of the work, 2k was for the turbo alone
big turbo big boost big powah!
heres a nice easy guide to explain the ease of this project download the pdf.
http://www.skyline-owners-club.com/foru ... guide.html
all comes down to how much you wanna spend how much you want out of it and how long you want it to last