Out here in SoCal, you can find Major Brand Name 14, 16, 18, & 20 speed road bikes priced anywhere from $175-$400 on CraigsList. There's a Trek 2100 (16-speed STI) Composite Carbon Fiber bike selling for $180, with an extra set of wheels and tires. I think it would be less expensive to buy a newer bike for the parts, than try to buy all the parts separately. The bonus is that you could ride the newer bike and see how you like it, before you strip it. You might find that you actually like it better than the old bike you're planning on upgrading.

Do you actually have a use for the extra gears, or are you planning an ungrade for the"cool" factor? I hate to break it to you, but nobody is counting the cogs on your rear wheel.