You'd definitely have to be closed for the sabbath. Also the week of Passover (plus if you are Jewish, there are some other technicalities involved with Passover, since you're not actually supposed to own anything that isn't kosher for passover. There are ways around this, usually a symbolic selling of your stuff for the week to a non-jew. You can ask the Rabbi about it).
There is no reason to be closed the entire week of Sukkoth, it is just the first two and last two days which you couldn't be open, as they have the same restrictions as the sabbath, in terms of conducting monetary transactions, operating electrical machines, etc.... Hannukah does not have these prohibitions, so you could definitely be open then.
There are several brands of kosher fondant, including Wilton (which tastes gross, IMO), Satin Ice, and Baekels.
Some great substitutes for milk are soy milk and rice milk. There is also non-dairy creamer (like Rich's), but it tends to have a lot of artificial ingredients, and doesn't taste so great. I almost always use soy milk to keep my baked goods non-dairy. Most kosher bakeries have the vast majority of their products pareve (hebrew term for non-dairy), so there are certainly ways to bake well without dairy.
There are different types of kosher certifications which are accepted by different people, at various levels of observance. If you want to make sure you're covering all your bases and making your business accessible to the largest number of kashruth observers, then your best bet is to consult a national organization like the Orthodox Union (www.ou.org
) or the Star K (http://www.star-k.org/
). They should both be able to guide you as to how to get started, what expense are involved (it will incur a cost to be certified), and answer other questions.
There are also many regional, local kosher certifying organizations and individual rabbis, but I would research which ones in your area are most widely accepted by kosher observant people, so that you'll get the most 'bang for your buck.'
One last suggestion: If there are other kosher establishments in your area (restaurants, delis, take out, etc...) I would see if their owners might be willing to talk to you and tell you what their experience has been dealing with the local kosher certifying agencies. There are a lot of intricacies involved, many of which can be an inconvenience. But you should gather your info and then decide if it's worth your while to pursue.
Hope that helps, best of luck!!