Something that is fairly easy to find or possibly upgrade components to? New or used doesn't matter. $1,000 or less target price range.
That would be the Bike Friday Tikit, which you can find for around $1000 if you peruse eBay and Craigslist. MUCH more expensive new.
Or don't fret so much about ultimate speed and just get a 6-speed Brompton with the 12% + ratio option. They're great bikes.