dude! i think you should buy the 05' YZ250, its the newest, and best bike you listed, and its worth the money.
BUT i would but the TWO YZF250, sell the one for "X" amount of money, and fix the other one and sell it for "X" amount of money, And if you sell the first one for enough (Like $2,000) or something all you would need to do is fix the other bike and sell it for at least a grand or more to make a decent profit. Depending whats wrong with it to. What does it need, just a new piston and cylinder, or does it need EVERY thing else? because if its just the piston, and a cylinder, that would be a STEAL! but if all the valves and the head needs replaced i wouldn't worry about it.