Bromptons are very good at what they do. Great design, ride well but quirky and perhaps no so great engineering. I wonder if I'm being churlish about my 25 year old Brompton when I say hardly a month goes by without something breaking. Maybe it's an age thing, but you have to worry about more than the obvious parts. For example, recently my front brake lever snapped while braking - presumably due to fatigue. Thankfully I did not die. In the last year, I've replaced seat sleeve x2, hub gear adjuster, chain tensioner and brake lever - all due to age. It's had two tyres and two tubes as well.
I say this not to put you off, but be aware that if you are after a cheap brompton, the chances are it will be old and even parts like handlebars might be at the end of their useful life. They are regarded as a consumable. Repairs due to proprietary parts is not cheap, but at least they are widely available.
A five year old brompton would be a good buy, an older one I'd be circumspect about it.
The Dahon above sounds good (I tried one and they are stiffer than a brompton), but I'd be suspicious about the spokes needing replacing. I've replaced one spoke in the 15 years I've had my brompton.