Planning to buy Grooming Clippers..

I volunteer at a local shelter and decided that bathing and grooming is where I need to be working at. So many of the animals are passed over for being dirty and matted. We have very limited supples and I am tired of used dull scissors. I would like to purchase a pair of grooming clipper but want to be sure I get the right ones. Since I am using my money can anyone give me some advise. Cord vs cordless and one brand over another??? Thanks a bunch
PS I try to do about 4-5 dogs in one day. I heard some clippers can get really hot and burn the dogs.