What to look for, IMO, is what you look for in any breed or variety. You look for birds the are representative of the defined standard for the breed. So what I would do first is get ahold of a copy of the Standard & study the description of the campine breed.
They are a beautiful bird. I've always thought they are one of the most beautiful. They tend to be flighty but are rarely aggressive. With some handlig they tame nicely. The strain I have are excellent layers. I bred from 4 females this year. I set weekly when I'm hatching & I never set less than 25 eggs in any given week. For several weeks I set the maximum possible 28 eggs.
They lay a good sized pure white egg. Mine lay eggs with an average weight of 53 grams which is between medium & large.