Generally speaking stock examples of Supra's are always a guaranteed sell and command a premium. Highly modified cars may be worth more in parts purchased for the car, but the value never translates properly into the actual selling price. This has been the case for the last 5-10 years of supra selling, I don't see it changing anytime soon, especially as 'mint' unmolested examples are becoming scarce.
If you modify the car, you have to take a bet that someone will like exactly what you did to the car and is willing to pay more for it. Sometimes the bet doesn't work out, and you see a lot of big spec cars being broken for parts instead.
If you are building the car just for you with no cares about future resale etc, then go balls out. If you are already having doubts about how it will sell in future, then poking holes in your chassis to mount a wide arch kit probably isn't the move for you.