There is a SR20det for sale on e-bay for $899. This is a common swap. There is a rb28det on ebay for $1200.
My $0.02: I would go with the GTR motor. The 2jz is more robust, but the skyline motor is a Nissan motor and it's a way more rare
. People have paid premium dollar for 240z from all generations to get the RB dropped in. I've seen so many SR20 swaps that I honestly ignore them.
With the RB you will be very likely to find a Nissan purist who will not be able live without it.