For sure, make sure there are no nails and screws left lying around, if you build the coop. I spent a whole afternoon checking our construction area and picking up the odd screw.
I don't feed them:
Avocado, banana skins, apple pips, raw potato, onions (though a little garlic is good for them), raw or dry beans, citrus, sweets, chocolate, anything with a lot of salt in it. Anything that is mouldy or off - if I wouldn't eat it, I don't give it to them. Raw egg, in case it gives them bad ideas. I have seen pumpkin seeds mentioned as not to be given to them, not sure why.
I have used this website, which lists toxic plants to be wary of, in case these are in your garden.
http://www.poultryhelp.com/toxicplants.html
If you buy a good quality feed and supplement that with greens and fruit, they will be happy!
Good luck with the exciting adventure
Katrina