Here are some of my thoughts. For the record... I only keep chickens as pets, but do eat the eggs. I also do eat chickens from the grocery store, as well as chicken out at KFC. I wont eat my pets.
Broilers are intended to be meat birds. They grow fast and do not get to egg laying status very often. Most are ready to eat at 8 weeks or so. You may raise them in a wilder fashion so they grow slower, but then your return $$$ is lowered. JMO. This is if you are doing it to sell. Dual purpose chickens are of course good. You turn them into meat at the end of their useful laying cycle.. (they will still lay, but the amount of eggs is no longer profitable compared to feed consumed) These chickens are not as tender as broilers.
The second part in your agreement with your customer will need to be the purchase amounts when your broilers mature. I am assuming that you would get broiler chicks from the hatchery at like 15 count batches. After 8 week, your customer would need to take ALL of them at same time. Broilers will not live long lives past their ideal harvest stage, and develop, broken bones, and also cardiac issues. (its not going to happen right away like week 10 or 14 but you get the point) You would not want to be selling broiler chickens that are on their last breath. You mentioned that you Luv your chickens. I understand where you stand.
Now if you are hatching your own chickens, You may consider selling your roosters to that customer. Wait until they are somewhat grown but not old and tough. It has been calculated by members here on BYC. that such operations are not cost effective. That translates to rooster eating more feed $ than his sale brings in $..
You have all these things to consider, as well as accommodating the additional chickens. You may be better off just staying with eggs.
Everyones situation is different so it may work for you.
WISHING YOU BEST.