:lol: I've heard of some of the best prelim work being done with lipstick on the back of a bar napkin.
Seriously, unless you're doing "art quality" sketches I don't think you need to worry too much. Just try different things until you find someting within your budget that you like working with. Whether notebook with a no. 2 pencil or posterboard with charcoal, it's only for you to work out ideas no? And even if you're "selling" the ideas to prospective clients, I doubt most would care much, though lipstick on bar napkins might not be the best choice