Your post interested me because I was in your position last year. BYC is the best source of information and help that you will find. Usually your questions will have already been asked and answered here, but if you get stumped everyone here is happy to help, it is amazing!
I went to the feed store for RIR, but ended up getting some Russian Orloff chicks on accident. So we went back a few weeks later for some more RIR and they had none, and I had been reading about Buff Orpingtons on BYC. So we got eight of them. We are mainly looking for eggs but wanted to be able to eat the extra cockerels so wanted a dual purpose breed.
We would now wholeheartedly vote Buff Orpington. They were always the friendly ones. They lay gorgeous pinkish-brownish large eggs. They do not fly away or escape or fight. One of ours is already broody and we want chicks. And finally, they are so pretty!
The RIR are very productive with egg laying however and lay beautiful eggs, probably a few more and layed younger than the BO. We do have a few of those that are friendly, but their personalities vary more. One is a real character.
We did find having three different breeds seems to make them get along better as they seem to like to be with their own breed. I had read that RIR are aggressive and expected them to rule the roost, but our largest BO hen seems to rule the roost. We did acquire a BO roo so that we can create more of them eventually as our breed of choice. But having a few types seemed to help them sort out their pecking order and know who their friends are. We have a very cohesive flock (by accident!). I am not sure what we have done right, but following the BYC experts advice has been our guiding light, I mean it!
Even though we did accidentely get the wrong breed shopping at the feed store, I was still happy with the experience. We like the Orloffs after all even if they are escape artists and it helps when you do head counts if they don't all look the same. We did get appropriate breeds for our area without even trying because the store offered birds that do well in our climate. I had read you will probably lose a few birds so bought extra, but we managed to raise all ours, I think partly because "shipping" consisted of coming home in a cardboard box on a 1/2 hour drive on my lap. We wanted about five to eight and ended up with 18, counting our roo. However we are getting over a dozen eggs every day. All three of our breeds are laying very well, other day we got 15 from our 17 females.
Welcome to having chickens, you will like it more than you expected! We are so proud of our eggs, you would think we laid them ourselves.