I bought my first Jazz last summer, for exactly the same reason as you did. I previously had a Volvo S40. The Jazz isn't as well insulated, noise wise, as the Volvo was, but I don't find that road noise is intrusive. It is not such a smooth ride as I was used to, but the shorter wheelbase makes that inevitable. Road noise is greatly influenced by the type of tyre fitted to the vehicle as, to a less extent, is ride comfort.
With regard to plugs, Iridium plugs are supposed to be good for up to 75K miles between changes, but if the plugs are just standard I would be inclined to change them.
I have fallen in love with my Jazz, over the past 10 months, and find that the savings in fuel alone, more than make up for any shortcomings!