My experience is with a Stagea RB25DET into a R32 GTS4.
The engine drops straight in. The gearbox bolts straight up, and is handling 180kw out through all 4 wheels (estimated 240 at the flywheel). You need to retain your (RB20) P/S pump (for the HICAS).
The NEO is probably similar to the Stagea, in that it has VVT. So, a RB20 PFC will not run it (doesn't know about VVT); best getting a PFC to suit the engine. Get the setup as a half-cut so you get the loom as well to use with your new PFC (you can possibly off-load the gearbox, brakes, etc to recoup some of your cost). A half cut will also give you an ECU, so no need to get the new PFC immediately. No idea on cost; probably 3-4k. Fit at least a HD clutch pressure plate; ceramic clutch and a lightweight flywheel are optional.
It's probably a day each to remove your engine and install the NEO. You will need an engine crane (hireable). There is a lot of "remove <something> to gain access to <some bolt>", so it's fiddly, time-consuming work.
Wiring in - step 1: remove front bar and LH guard. (I'm not kidding!)
As for your current engine, probably not really worth much at all. They are dime-a-dozen from wreckers. You might get 1k if you were really lucky.
Hope that gives you some idea of what's involved. It's certainly worthwhile - the RB25 has torque to burn!