Bromptons are no longer rare, I think they upped their production in recent years by 7-8X. So there is no need to pay anywhere close to new price for a used one, and every reason not to, because you can't know the condition of the internal gear hub at purchase, and service of them is expensive, when you can find a shop to do so.

You don't need huge gear range, I think 400% gearing is sufficient, 21 gear inch low will get up up most hills without a big cargo load, and 85 gear inches is high enough to pedal down gentle grades, and steeper than that you just coast. Looking online now, it says Brompton 6 speeds are 300% gearing, I don't know exactly but I think about 30-90 gear inches. That's about what my Dahon Speed D7 came with, and it wasn't enough for hilly cities like SF and Seattle, I upgraded it to 400% and it's the best thing I ever did, should have done years earlier.

20" wheel bifolds (fold in half at the frame) are common and good cost, many on here have raved about the Zizzo Liberte and other models, but like my Dahon, have limited gearing. They also don't fold as small as a Brompton C-Line (16"/349 wheels). If your car is like a Miata NA (first model), I don't think a bifold will fit in the trunk. Do tell.

There are many Brompton clones at good prices in Asia, it's the shipping here at $300 cost that bumps the price up. Brompton has a good reputation for quality, so for a clone (aka brompnot), I personally would not pay more than 1/3 the cost of the genuine original.

But first, determine your gearing needs, by checking the gearing specs on a normal bike.

Second, what are your braking needs? Do rim brakes work for your riding conditions and terrain, or are your other bikes disc brakes?