I heard duck eggs are phenomenal for baking. My question is do ducks lay year around like most hens or are they seasonal? Where would you suggest I purchase a baby duck from? How old before you can tell their sex and how old before they start laying? Thanks for any help shared!
Frequency of Laying: that is dependent on the breed. I have Campbells and they are very good layers, but then I did buy/hatch them for that reason

Indian Runners are also good layers.
Buying: I'm in England so I cannot recommend anywhere for you.
Sexing: By six weeks you will know by the sounds they make; a female duck quacks - a male sounds like he has a sore throat. Some people are able to "sex" ducklings at one day old, but that takes a bit of experience and it is not something I have enough experience of.
Coming into Lay: The "average" is 20 weeks, but one of mine laid her first egg at 15 weeks and another waited until around the 25 week mark. Keep 20 weeks in your mind, but don't be surprised if you have to wait longer
Baking: yes, they are great for baking; but they can be substituted for *any* recipe that you would use a chicken egg for.
Drakes: I keep drakes for a number of reasons: 1) they take care of the girls; 2) they are quieter than girls, so why not; 3) my eggs are fertilised so, I can replenish my "stock" naturally. Accidents happen and ducks die, so, for me, I like that I can hatch a couple of eggs if I want to

But, that is very much a personal point of view and one not everyone shares.
Generally:
We have enormous fun with the ducks in the Spring/Summer and are less "trouble" than my chooks.
However, we have had rain almost contantly since October and now it is another ball game!!! Despite having a large area to "play" in, if it is not waterlogged (which the quackers love!) then it is one big mud pitch. This isn't a problem, but, it does make the daily chores a bit more of a "chore" and any mud they walk/sit in is quickly absorbed in their bedding which needs changing pretty much daily in this weather!! So, Autumn/Winter, chooks are less "trouble" than the quackers.
I wouldn't be without either though