You can use the same tranny, u don't need a new ecu but it won't run full potential . . U would need either a reflash of ur existing computer (ecu) Jim wolf technology does that or u can do the famous nismotronic or just a full standalone. U don't need a new wiring harness but u would need to swap over some sensors from the g20 sr20de to the sr20ve nothing major. But it's pretty straight forward. Good luck brother and GOD BLESS.
If it comes with ecu u wouldn't need to flash it for it is made for the ve the g20 or sentra models are not made for ve but for de so the tune or map on the stock de computer is notbthevsame as the ve. U can later do a tune on it for maybe u change intake headers and maybe injectors or what not then u can get it tuned for ur upgrades. But untill than u don't need to flash the factory sr20ve comp.
You'll also need something to control the VVL cams. I'm in the middle of doing everything you're talking about... I went with a Greddy MSS to control the cams, and an upgraded stock 1996 Manual G20 ECU that was upgraded with the proper fuel/spark maps for the SR20VE. It's also got the rev limiter disabled so I can get any extra power that may exist up to whatever RPM I'm brave enough to tickle. Maybe 8k with the N1 cams... but I guess we'll see
I know I'm really, really late to the game with VE swaps in a P10 - but since I made a wiring diagram for myself while working on the MSS wiring for my SR20VE project over the Christmas break, I thought I'd share incase anyone else needs this in the future: