Ok in terms of power, the GTR is tops.
R32:
Can never go wrong with an R32 GTR. As you said, it requires money to restore the car as it is pretty old. When things break, they can be expensive. Quick Car.
R34:
Similiar with the R34 GT-T. When things break, they are expensive. Lots of potential for bigger power. More space. Newer.
S15:
New Car. You might find things coming lose in the interior and hear noises in the cabin. Nice sleek look. 4 cycl so better fuel economy. Wont need to spend too much to get more power. Tail kicks out easy