1. You could go to 18's all around and user a taller(more sidewall) tire to keep the overall diameter the same on the back. That way the speedometer wouldn't be affected. I'm not sure about 17's on the front. You'd have to make sure the wheel would clear the brakes.
3. Going to a smaller wheel and higher profile tire will allow more flex in the sidewall which will adversely affect handling. It probably wouldn't affect it enough to make much difference in normal driving though.