As far as chick mortality rate goes, a lot of factors are depndant on you, and many are not. I have never raised a large number of birds, living in a suburban environment, but of all the ones I have had, I think Ive only lost 2-3. I can't tell you how many you should "expect" to lose, but I would tell you to not "expect" to lose any. Meaning, I would not go out and buy 10 chicks because if they all make it, you not only have 2 "illegal" chickens, you may not have enough room! I think 6 would be a good number to start with, so if you lose one or, you still have 4 or 5, which is your preferred number, and if they all make it, you do not run into the "illegal chicken" problem and your will likely have plenty of room for your extra hen! And remember, you can always add more, but sometimes its not as easy to take some away. I would also get all pullets unless you really want a rooster, and even if you do, you may not be allowed one where you live.
Now on to the fun part! Picking breeds! Im going to tell you to get Brahmas because they are my favorite and I'm incredibly biased, haha. Buff Orpingtons are a great breed, I do not have any personal expierience with owning them, but thats all my aunt ever raised becaused they are good layers and exceptionaly tolerant of the cold weather here in Colorado. Brahmas too, are almost completely immune to cold weather, but I've heard that some people think that their egg laying leaves something to be desired since they are such a massive breed. I never had any problems with egg production from them, though. Brahmas are also VERY friendly and one of mine was more like an affectionate house cat then a barnyard chicken. Another good breed that I have owned would be Wyandottes. They are great layers and very docile as well, from my expierience. They also come in many more color varieties then Buff Opringtons (as the name suggests, they are simply Buff...Orpingtons.) if that matters at all to you. All the breeds would do well I think in your part of the country since I think your main concern would be the cold? As long as you have good shade and plenty of water, summer heat shouldn't bother any of them either unless you live in Phoenix or, say, Baghdad.