Remember to quarantine them away from your current chickens for a couple weeks! During that time if you can take a poop sample to a vet and have them look for parasites, that would be wise.
At the same time, you could check a poop sample of your flock for parasites.
Then you won't be surprised by any parasitic illnesses. You will have to observe for any airborne illness over a period of time. Keep them on separate ground and in separate areas so that airborne pathogens cannot infect your flock!
Some folks keep the new birds on cement. That way if the new hens have an internal parasite, you can chlorox/bleach the cement after they are treated.