Some cheap applicators will mark your paintwork. For the sake of a few pence, I'd stick to decent ones. Tesco do a good 3 pack for £3.
A good way to test effectiveness, is to rub any applicators on a blank CD. If they scratch it, then they could well scratch your clearcoat, so are no good.
You can clean them out, I normally use Washing Up Liquid, but rarely do so, especially on other peoples' cars.
Things like tyre dressing applicators can be re-used though.