i think i would go for 185/45/15 or 185/40/15 if possible, where can i get these tyres from, as i asked my local tyre place he said 185/55/15 is the lowest profile there is!!
nankang do the NS-2 in a 185/45/15 and they are only £31 a corner.... but they arent know as nankang hedge finders (in fiat circles anyway) for no reason!ÂÂÂ

also... pirelli do the p6 in a 175/55/15 which are about £62 a corner.
and if your feeling rather mental, kumho and hankook do tyres in a 175/50/15, ranging from about £40 - £51 a cornerÂÂÂ

EDIT: the seicento euro boys frequently run 175/50/13 tyres on a 7x13 wheel, so i cant see it bein a problem on a 15" wheel if u want to go for that stretched euro lookÂÂÂ

and according to
http://www.carbibles.com/tyre_bible.html the 175's are 1.57mm smaller in overall diameter than the 195/45's as wellÂÂÂ