I just bought some chicks and guineas at the feed store today... A lot depends on where you live, too. You don't want to buy chicks in October if you live in a northern state because then you would be looking at having chicks in your house all winter. But here in North Dakota, I have bought chicks in early September and they were fine when the snow flew.
If you keep their pen clean, there should be no problem with smell. I pile it into a compost pile and it later goes into the garden. Just make sure that you have everything ready before you get any. It isn't any fun when they need to be moved out of the brooder and you don't have the pen ready yet! I don't know where you live, but if there are predators there, you need to consider that when you make their house. For instance, if you have racoons, you need a covered pen. It's devastating when you go out and find that your beautiful birds have all been torn apart!
Good luck, they really are wonderful to have!