Taxonomy vs Field questions

I am attempting to conquer custompress this week. My question is regarding structure. When would I use taxonomy vs. a field? for example my product is yachts. It is similar to real-estate..... or even automobiles in the way searching and sorting needs to occur for the public view. But i am confused how to start. Can a field be converted to a taxonomy later? Can fields be searched and sorted? Do i need taxonomy at all?

Thanks in advance.