Go to www.murraymcmurrayhatchery.com Or someting like that or you can go to google.com and type in mcmurray. They have pictures of grown up and baby chickens.
I would personaly recommend chicks. It is a great experiance and They will learn to trust you early on.
If you put the adult chickens with the chicks you will find dead chicks everywere!!! Orpingtons are good egg layers and are very friendly.
Have a nice game!
CFC
It is normal for wild birds to be in your flock. They are just looking for food.
They can give diseses to your flock though. Oh and remember that wild birds have mites and lice.
To keep the wild birds out all that you really can do is put a top on you run.
Hoped I helped,
CFC