The idea of going with a non-turbo may sound appealing at first, but in the end I think it will probably be more trouble than it's worth. Even if you're going to ditch the sequential turbos, you still have the plumbing, piping, and ECU changeout to sort though. (Someone feel free to jump in if I'm incorrect about the ECU issue. The non- turbo ECU can't support a turbo motor, can it?)
Were you planning to try and run 700hp on stock internals? If you plan to drive this car on the streets regularly, you are pushing the reliability limits of the turbo motor. I seriously doubt the non-turbo could be pushed anywhere near this limit due to internal differences, the most significant of which includes a higher compression ratio. Also keep in mind that a non-turbo model sports a five speed manual; only turbo models received the six speed.
Speaking of transmissions, are you planning to go with an automatic transmission or a manual? The auto Supra is very solid, and my research has found that you can now buy nice turbo automatics for somewhat reasonable prices (mid twenties), which is not a ton more than a nicely kept manual non-turbo.
Long story short: Unless you really plan on tearing into this thing and going far beyond bolt on components, you are probably better off starting with the turbo.
Then again, if you planning on building the bottom end of what you buy, the Supra lose some of its appeal. The main appeal of the Supra is its bullet proof internals, but if you plan on providing your own internal reinforcements, 300ZXs, 3000gts, and RX7s suddenly come to view as having nearly as much potential as the Supra.
If the Supra just does it for you, though, go for it. There's not a thing wrong with spending money on the car that you really like.