I have buff orpingtons in my beginner flock (as well as welsummers, wyandottes, Easter Eggers/Ameracanas, silkies, polish and mille fleurs!). Of the three, only one is really friendly. That one is superduper friendly. She flys up to my lap every evening when I sit in the run. She just settles down, fluffs out and chirps quietly to herself. Such a sweetie! The orps have lots of really fluffy feathers. She's very soft. The other chickens have feathers that lay down along their bodies. The buff orps all stand out a bit. They look about twice as big as their bodies really are.
At least one of the other two is a rooster. I can't tell yet on the third. The rooster and third are both a little stand-offish. They never purposely fly up into my lap. If I snag them, they will eventually settle down and let me keep a hand on them. But they are not especially friendly.
Here's a pic of the rooster at age 5.5 weeks-
My wyandottes are busy little chickens. They will fly up to see what I"m doing (nosy!) but they never really stick around or want to be petted. They're fun to watch though and very pretty.
The friendliest two in my lot are the buff orp hen and the mille fleur rooster. I can count on both to sit for a while in the evening. The others kind of come and go. The shyest three are buff orps and an Easter Egger. All 17 chicks were handled daily from day one. I think it just comes down to individual personality.
I have no experience with Australorps or Brahmas (although my mille fleurs and silkies have feathered feet). I too recommend getting a mix initially so you can see which ones you like best. I have found that the ones I enjoy most right now are not the ones I thought I would. Also, I didn't order any roosters and I ended up with two, perhaps three. You just never really know what you're going to get unless you get them as mostly grown chickens.
Good luck! I just got into it too after years of research. You're going to have a blast!