The SR20VET engine in the Xtrail is AWD ready. It's expensive and has a turbo and lower static compression that would have to be dealt with, but it would be a great starting point if you can afford it. If you're going to do AWD you need a "narrow" SR20 block from an AWD or AWD-ready car like the Bluebird I believe (or obviously the GTi-R).
Personally I think RWD is a much better idea.
For RWD like everyone else has said, you'll likely want to start with a RWD SR20DE or SR20DET engine (so the transmission will bolt up) and swap on the VE head by doing the work involved.
It's a long thread, but there's some info on what is involved in this conversion in this thread:
http://www.sr20-forum.com/general-sr20/9718-how-build-300-whp-liter-sr-cheap-discussion.html