As far as the debate between fully build KA's and RB's, price wise here is something I have found interesting.
RB25DET half cut, with motor, tranny, driveshaft, and fuel pump ( read somewhere else it also includes ecu, but not sure because its not listed, just those four are) will run you $3,396.
Swap kit for use with the R33 mounts (from the clip) $585
There is your RB25DET swap right there. Super Street has an article about this swap, stock RB25DET in a 240 at 306 HP and 246 torque, on SR Motorcars' dynojet.
So, that right there looks about $3,981. Add about $200 or so for random parts (driveshaft, fans, exhaust, etc) and shipping and such, and you have a pricetag of $5,481.
Now, for the KA longblock from AMS, your price tag is $4,059. That doesn't include your manifolds, tranny, clutch, oil pan, valve cover, turbo, or anything. Toss in at least 2500 for all that (gotta get all the best, or why else spend 4 grand on a 4 cyl longblock then cheap the rest?) That would run you right about $6,559.
Now, I could go look up every piece of accessory you will need and add them up, chances are they would be even more than 2500 together. I am not going to do that though, just giving a rough estimate. That is just to get you on your KA to the RB equivalent.
So, using this information, and the fact that you would be putting less stress on a 6 cyl to get 300 hp than a 4 cyl, and the fact that the rb25 has more future modding capability than the KA, I would say the RB is your best bet...
Unless you don't have the 5 grand or so for the swap, then just slap a turbo on your current motor and hope for the best.