The first thing I'll say is get a drive in a single turbo Supra. It's a whole different ballgame to a TT.
I've had a Supra in every stage up to medium-sized single. I've had stock, BPU, BPU with hybrids, TTC, small single turbo and now my modest Borg Warner.
I suppose being 'worth the money' is a completely personal thing. I don't mind spending £50k on a car that is faster and more involving to drive than pretty much anything out there (in a straight line at least) but obviously there has to be a limit. I've done my modifications over five years, so didn't take the hit at once. If you're looking at fully building a Supra into a built engine, big turbo car in one go then I think your money may well be better spent buying something faster to begin with and playing with that. 996 911 turbo would be my choice.
If you're set on a big power Supra, my advice would be to do it in stages and end up with a reliable and fast car. 700bhp isn't an easy project, and you'll have to do a lot of work to build a car that can support it reliably.