I usually buy a three piece kit, friction plate, release bearing and pressure plate then fit the lot along with a new spigot bush. that bush costs pennies so is worth doing whilst everything's out.
You can reuse the pressure plate as long as there's no spring fingers missing or damaged, there's no excessive scoring where the release bearing pushes on them and there's no heavy scoring on the drive face. The friction plate can be reused if there's nothing obvious wrong with it, ditto the release bearing. But! considering the price of the parts compared to you having to pull the gearbox out a second time to change what ever part you didnt change before, you might as well change the lot.
Hope that's of some help!
Rob.
