Peppered Cories do better in a tank with cooler temperatures and Panda Cories are very sensitive.
Keep an eye out at your lfs to see what variety they have.
1. would they be good to keep a sand tank reasonably clean
They spend most of their time scavenging leftover food that other fish have left.
2. how much do they cost
Prices vary, anywhere between £1.50 - £10.00
3. how many should i get to keep them happy
They prefer to be kept in a group of 5+
4.do they need anything special in the tank eg plants
I don't know that they necessarily need plants, although I do find mine sitting on the leaves sometimes! They do like some hiding places though as they can be a little shy.
5. what should i feed them
They aren't overly fussy! Mine eat frozen food, catfish pellets and flakes.