Yup, rosemarythyme brings up an excellent point. Aging hens are going to happen quickly especially if you are planning on using production birds. They age quicker due to the production of so many eggs per year than the heritage birds and then you are going to have to have a plan for dealing with them.
There are no difference between the economic reality of "Heritage" hens and commercial hens. I say this because both types of hens are on the downward slope from the minute they hatch. The only difference is that you will not lose as much money as fast with production hens as you stand to lose with "Heritage hens" mostly because you'll need to feed the inefficient Heritage hens for a much longer period of time before they get to the break even point.
There is a lot of things that goes into producing table eggs than meets the eye. You will need replacement birds, new or improved housing, etc,etc, etc. This is where depreciation comes into play.
Also to raise 300 replacement pullets you'll need a minimum of 600 unsexed baby chicks and even the best of us will lose maybe 10% of our replacement birds for one reason or another. Therefore make that 660 unsexed baby chicks per year. @ $5 per chick that equals $3300 Green Backs every 12 months or a dead expense of $280 per month. Of course you can recapture some of this outlay if you slaughter your Spring Chickens but can you bring yourself to do so even in the face of your family going hungry. Or sleeping on the street.
300 producing hens quartered on 1/4 of an acre with two paddocks equals about 11&1/2 square feet per productive hen on 1/8th of an acre. This will hardly produce any useful forage for your hens. Now once the gearatic hens and your young stuff is somehow figured into the equation not to mention those 100s & 100s of boy hens I don't see how your chickens can keep from stepping on each other because they will be jammed packed together like sardines, not chickens..
I don't intend to throw cold water on your dreams because in a free country dreams are what propels the economy What I am saying is don't quit your day job.