I think single breed or mixed flock is totally up to you. I wanted a diverse colorful looking flock so I went with different breed but went with all cold hardy (I'm in MA), docile, and good laying breeds. If you are thinking of breeding then it seems to me a single breed would make it easier without having to keep breeds separated.
As for hatchery or breeder. One deciding factor might be if you decide to do a flock of different breeds. Maybe easier to get a bunch of different breeds from a hatchery as opposed to finding breeders of each of those breeds with chicks hatching at the same time. I actually have 3 hatchery chicks (got 4 but one died) and then added 2 more from a local breeder. To my newbie totally inexperienced eyes I can't tell the difference. Not planning on showing (unless my kids decide to do 4H) so I am more focused on personality type and good layers. (Oh all my chicks also happen to be dual purpose breeds in addition to docile, cold hardy, and good layers).
If you are thinking of other breeds there seem like there would be a lot that would fit what you are looking for. I think My Pet
Chicken has a "What breed is right for me"
kind of tool.