Well, let's see, I personally have never heard of a 750HP Toyota Supra. If this is a lowered 750HP '94 Supra then you need to consider a few things that are absolutely necessary (in my opinion):
1- Is this a well-functioning Supra? Anything over 400HP for a normal Supra is a bit too much, which means several things have to be updated and upgraded in order for the car to be able to handle 750HP, which means that the Price should be much higher considering the fact that so many things must have been upgraded or added to it. Is there anything wrong with it?
2- Is the price good for a Modified used '94 Supra? Seeing as to how it's got nearly 70000Miles on it, it might means that some components may have reached their lifetime limit. Did the car do 70000 Miles after it was tweaked to 750HP, or before? If it went 70000 Miles after the modification chances are you've got yourself an over-worked horse that did all it's work in 5th or 6th gear (but I guess it could go either way....)
3- Have you test-driven it? I'm skeptical as to whether or not a lowered 750+HP Supra can handle smoothly (or handle at all). Check to see if the previous owner is still alive. If so, I think you should insist that you take it for a test-drive or something.
4- I guess you should make sure the suspension and grip, and the brakes have been upgraded for a 750HP level car (of course judging by the $2900 brembos I'd say that's already in order).
5- Consider the fact that the overall costs of maintenance for such a high performance car is usually insanely high. Also there's the very real possibility that this car is a major gass-guzzler that uses up a full tank before you even drive the car home from where you bought it (and with those gas prices lately...). I'm saying this because it doesn't sound like you're loaded (and most of us aren't). But if you don't care about these costs then party on!
6- Lastly, make sure your car doesn't fly into orbit when you hit 6th gear!
That's about all I can think of for now. I hope it helps. I wrote most of the above thinking that you didn't already consider them (seeing as to how you're a "new sports car buyer". That's about it for now.
P.S.: That's a phat car! :P
'94 Toyota Supra Specs; To get or not to get 
I already looked into this car, you can't get a Supra with many more mods than this. And many Supras have been known to push 800 bhp without any internal engine mods (i.e. no balancing or blueprinting), which is a great attestament to the engineering put into the engine. Either way, like any blown engine, it will be prone to detonation unless you make ABSOLUTELY sure it is VERY well cooler, intercooled, etc.
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