It's far easier to bring in/raise a full flock at once, however one thing to keep in mind is that egg production will start to drop off as early as 2 years. For that reason many keepers have flocks with staggered ages, to keep egg production rolling along more smoothly.
You'll still have downtime especially if you don't provide light in the winter (I haven't had eggs since October or so, as my hens are over 2 and molting and my pullets are still too young) but once they start up the younger birds should help with production.
As a continuation from above, the reason it's easier to bring in a flock all at once is because chickens are territorial and adding new chickens can be a real headache. The flock can attack and injure birds they don't recognize. Integrating new birds requires TIME and SPACE - even introducing chicks to a mature flock (the "easiest" integration) can take several weeks. With your current planned setup you may find it tight space wise for integrating birds, as it helps to have room to put in extra feeders, waterers, and obstacles to allow new birds to get away from bullies.
That said, I haven't had issue adding birds thus far but I've only ever added chicks with adults and pullets with pullets the same age.
Would you be interested in starting with chicks or are you interested in older birds only? Obvious advantage with chicks is they're cute, easier to source (you can order from a hatchery or go to a local feed store or breeder), and if you handle them from the start they're more likely to be friendly as adults. Downside is they're obviously a little more time consuming and depending on breed(s) chosen and source there's the possibility that you'll get a male.
As far as breeds, IF you want to start with chicks and want to make 100% sure you don't get males, then any of the sex-linked types would be an option. If you're not a fan of high production hybrids then I'd recommend having at least one Easter Egger, as they come in almost every possible color and their eggs come in a variety of colors too.