The Buggy: not a character itself, but shown very often since it is used by the kids and cousin to explore Auckland. Due to this frequency, I'd give it 3 stars.

The Black Chevy: this one is more distinctive as a "mothership" for the slimy monsters, and it is shown very often too, but it is not the absolute star at all (not Herbie, I mean), so I'd give it a 3-4 stars.

Both of the cars are shown in most 8 episodes, but more frequently from 1st thru 4th.

Hope this helps.