googlebomber, i was just starting off doing the same thing myself.

Really, what I am doing at the moment is getting supplier catalogue's from the place i work, and pitching the prices against other national retailers - finding out the profit, and then finding out how many customers you would be expecting/needing.

from what i can see, it's no harm to make estimates once they're backed by proof. also, business plans are just as much about setting up goals as they are about how to achieve them.

definite SEO figures will not be known until you are well into the course of your online store. the best i could recommend you to do is just to say "over the course of a 12 month period, i would like to have search engine visitors to increase by 25%"