Where to find classification and data structure advice

we are in the process of developing a classified ad website due to launch in about a month (yeah i know, this overcrowded market, but we plan to focus on a certain region which is currently under-served and plan to develop that specific market by combining various off-line and on-line methods)

and would need some good advice on how to structure the categories (of course this would come down to the market we are looking at but what i am seeking is a sort of general repository or advice) and also what kind of data that should go in each ad to be posted

i have looked around at 20+ websites all from the simple title and description settings to the viciously structured ones with lots of choices for the user to fill in

so anything that would come to mind or references to such advices would be great