Loads of Chinese tackle on ebay for that money - but not necessarily with all the things you're looking for, like suspension (tricky) or a larger battery for 35/40 miles - or if a larger battery, a quick-ish charge. I suspect you'll need to compromise somewhere. Most have cable disc brakes as standard these days.
Most folders, being small wheeled, are already thrashing nearly all of the RPM out of the motor as it is - derestricting (if possible) gets you a few extra MPH at best.
A good place to look is
www.markshop.co.uk for the simple reason he stocks and sells most of the popular/better selling eastern-made (i.e. at your price level) folding bike brands such as Himo, A20, Fiido, Samebike. Most importantly, he's one of the few UK resellers that will actually service/fix them too if/when they break - otherwise you could be on your own with zero support.
Otherwise, Woosh's Rambletta - for value and support - if there's stock, and you can stretch your budget a little.