I did the same thing I wanted to upgrade to a rb26 instead of the rb25 and it has cost me in the excess of $20k, this is what you should do if you go for the rb26..
Cost of engine $3000-$5500 (please make sure they supply comp test and op test & a warranty if possible)
Cost of head gasket from Nissan $80.00 this is so you can pull it down and inspect, there is nothing wrong with genuine head gaskets. This is where it cost me so much, engine was sold to me as a good runner but we pulled it down and found damage to the block and pistons so complete rebuild.
Rewire of wiring harness to run rb26 computer $200-$600 (depending on where you go)
Computer $300-$5500 (depending on where you go & what you use)
Modify intercooler plumbing $150-$500 again (depending on where you go)
Modification to sump $300-800 (depending on where you go)
If you want to make power at high rpm a good oil pump and coller is recomended, can go into the whole debate here but an N1 is a good start its supperior to a standard item to start with.
A good harmonic ballancer ATI or ROSS most profer ATI as do I.
Oil control, while modifing the sump ensure extra compacity and baffles maybe even a return from the back of the head to sump.
If you are staying with the twins make sure you change the dump pipes while the engine is out of the car likewise if changing the turbos. (its hard enough doing it when the engine is out let alone in the car)
Replace all heater/vacume/oil lines under intake plenum while engine is out too this is for peice of mind.
I think staying the the rb25 and upgrading the rob bolts to start with is a much wiser choice and will be cheaper in the end. I could keep going but hopefully this will be some help to you.