$10k is very little to buy an RB26 and install it then have an amazing car ontop that. In other words, you'd have not nearly enough money for good suspension, brakes, paint, wheels/tires, interior, etc.
If you'd said you had at least $20k to work with, then it'd be a little more doable.
I may be missing something, but it seems very strange that the price for the install with the 5spd RWD tranny is more expensive. You can get a tranny from a Z32 and bolt it right up, last I checked, and that's only $500. But then you gotta modify the oil pan anyway.
That shop you've referenced is charging $6k for labor alone, right?