Paul+Kerry,
As to whether the 18's will be a good bet - it depends on your present tyre/rim combo's 'rolling circumference' size?
Rolling circumference (RC) is of the length of the tyre's revolution, taken from a singular point on the tyre's surface, all around the outside face of the tread face, back to that single point again?
If the RC is the same as that of the proposed 18 inch alloy tyre/rim combo, then for you, it's game on pal!
What you need to bear in mind is that, if the RC of the 18" alloys is larger than that of your present set-up, then this will affect your speedo - the larger tyre/rim combo would, in effect, make it so you are actually going 'faster' than the speed which indicated on the clock?
IE: the clock tells you that you are moving at say, 30 mph, but in reality, you may actually be going at (lets assume) something like 37 mph - not good at all if you are in a 30 zone, eh!?
Better to check that out before you commit to buying them 18's bud.
However, if what i am told is correct, one can get the speedo re-calibrated to match up with the new size rim/tyre combo - i am not 100% sure of this, so it may well not be the case. But a bit of research should turn up some info on it.
Hope this helps you out?
Regards,
Grim