If the voice is raspy then he's mostly likely a drake. You will need to separate him from the chickens if he's trying to breed them. Their reproductive systems are very different and a drake can fatally injure a hen trying to mount her.
10 dollars a dozen sounds normal for my area when it comes to duck eggs now.
Please make sure to look into your city's regulations before selling any eggs. Some areas are much more strict with their rules when it comes to selling them.
I believe the protein enzymes are different in each species. Your digestive system might have trouble breaking them down.
This isn't a completely unusual reaction to them. I have a few family members who aren't able to eat duck eggs but are perfectly fine with chicken eggs.
It sounds like they might be trying to restablish their pecking order.
I would separate the injured male while he recovers.
It is possible for them to kill him but it usually doesn't end up going that far.
It really is going to depend on what you plan on stocking the tank with. A fish only tank will require a different set up than if you were planning on building a reef set up.
I wouldn't say it's really much harder, especially if you're going for a fish only tank or even a FOWLR.