I'm actually looking into more than 700WHP. I also like the wide body and the loook of the wider wheels an tires. I'm will only lower the car as much is needed for optimum look and performance. I'm going to work with the shop after the body work is done to get the right offsets and wheel measurements. I'm thinking at least 800WHP. My original plan was 900WHP and that's more what I'm looking for.

I'm currently running a twin turbo engine in a N/A body with a stock 5 speed transmission and N/A rear end. I've got 400WHP with 410lbft of torque according to the dyno. I'm currently running it on premium pump gas.

What's the best way to get to 900WHP from where I am right now? Do I need to upgrade to the 6 speed Getrag transmission and a different rear end to handle the power? Most performance shops are recommending a total rebuild from the bottom up. My understanding is that I should be able to meet my goals without touching the internals. Any thoughts on this?

Yep the oem n/a 5spd won't handle 900whp, so the 6spd is a must. I don't have max hp specs on the n/a diff, but I'm about 95%+ certain you'll have to change that too.
You could run 850rwhp (6spd) reliably with the oem 2jz-gte bottom end. 900rwhp is pushing it, imo. Of course you'll need 272 cams (preferrably HKS)...and some porting on the head wouldn't hurt either. If you're going to get it ported, make sure it's by someone who can prove their results on the 2jz-gte. A bad porting job will make worse power than an oem head.