handling car 17's or 15's?

The point about having the suspension rebuilt/upgraded first is right on.

But I think 17's are the best choice at the moment for balancing size, weight and selection for both rims and tires. 16's would be a better rim size and profile, but tire and rim selection is very limited. 18's are getting a little big, the rims get heavy because of their size, and you need some sidewall for traction.

It's about balance, and with the current technology and selection 17's probably give you the best options.