C5 Corvette. Why? U'll get one with low mileage. (MKIV's have 80K plus miles nowadays and are way overpriced - at the end of the day, in the eyes of automotive enthusiasts, "its just a Toyota - Jap crap" - there is no heritage, or significant automotive history that comes with any Supra. Who cares? We know the MKIV will put out one million horsepower and run forever, (so can the MKIII), but my point is $20K+ for a 80K+ mile Toyota is worthy of considering other cars with significantly lower mileage - my first thought is the American icon. 340 hp (?) stock and u'll get lower than half the mileage of a MKIV, and you have a car that appreciates worldwide status = money well spent. Put comparable mod money into it, and u'll have an all-motor monster.
I'm not even discussing a MKIII buildup unless you plan on restoring the entire car. We're so into modding our cars, but what about body work, and structural components? The MKIII requires collector car care, w/o collector car status. Bottom line = money pit. I love the MKIII, and think it is a great value for the money. We can enjoy 400rwhp for way less than the cost of a MKIV or C5. I love the MKIV, but can't justify its high mileage/cost ratio. But its really your decision. I would seriously consider a Vette before shelling out $20+ on a high mileage Toyota. Either way, good luck!