If buying used, to watch DVD's, mild websurfing and email, then you would probably get away with a p3~650. If using XP, then get as much memory as you can. 256mb should suffice... for now.
I have an IBM pII laptop that plays DVD's fine, but hesitate to recommend buying something so old. In fact, I won't.
I think we can give you better suggestions if we knew your rough budget on this purchase.