RB30 with 25de head would be the most feasible way making power without excessive amounts of work and extra parts. This option you can build the motor on the side and keep on driving your car right up until it's ready to go in. The rb30 bottom ends are fairly hardy so you may be able to get away with just new rings, bearing, water pump etc. just small inexpensive stuff.
The only problem with the rb30 conversion is the need for a stronger gearbox, which would probably be needed with a rb25 anyway. A New turbo will also be needed, so I suppose that's another cost to take into consideration.