the nice thing with the fiat 500 is it looks good with most rims. just depends on what look are you going for? just nothing with too much chrome and you should be fine. if youre going just for looks then 18" rims will be fine but if you want something more pratical and better handling then just stick with 17".
Wouldn't the HREs be worth more than the car itself in a couple of years? I like the TE37 in 17" idea; the key deciding factor for this decision should be weight, imho - the engine needs all the help it can get.